Five hundred people replied to the advert. "Have you ever dreamt of being a ballet-dancer but feel your size holds you back?" it asked. "Have you ever imagined dancing on a big stage in front of an adoring crowd? We are looking for talented dancers aged 18-55 to take part in our new series about ballet. No previous ballet experience necessary. If you're interested to hear more, we'd love to talk to you!"

Hannah Baines, an 18-year-old council administrator from Doncaster, knew immediately she wanted in. "I can dance, I know I can. I had just been put off by my weight," she said, after making it through the auditions for the new Channel 4 show Big Ballet last October. Wayne Sleep and Irish prima ballerina Monica Loughman, who handpicked the troupe, clearly agree.

"Just because I'm big doesn't lessen my dancing ability," she told me, standing in third position (one foot daintily in front of the other) as she waited for her dress-fitting. "There's no movement I can't do." Baines was with the wardrobe department of the Northern Ballet in Leeds, about to rehearse what the programme's voiceover (provided by Olivia Colman) would later dub a "ginormous performance" of Swan Lake. She thought for a while before correcting herself: "Obviously, I can't get my leg right up to my ear because my flab restricts me. But I'm pretty flexible. There's this misconception that dancing is about steps. It's not. It's about emotion, feeling."

It was hard not to feel nervous for this sweet young girl as she talked of her excitement at being picked to dance the lead. "It's my moment to shine. Everyone who's laughed at me, they're going to watch and see what I can do."

Baines is a size 18 and weighs 14 stone despite being just 5ft 3in tall. It was impossible not to worry about what would happen to her in the sometimes brutal world of reality TV. Would viewers tune in to laugh and point at someone playing a role usually danced by a woman half her size? How would she feel about being defined so publicly by her weight? Many of the dancers had spent their whole lives as the butt of a joke. One woman talked about attending an audition and being told: "WeightWatchers is this way." Many gave up dancing altogether decades ago, after being constantly told there was no part for them.

Early on in filming, Baines and her fellow Big Ballet stars got a taste of what the attention might be like, when Sleep gave an interview declaring them "quite frankly fat. They're too big to be dancers and they don't mind me saying it." Some of the women begged to differ and were quite upset. "I think he could have chosen better words, don't you?" said one. "More like, 'These ladies are real women and they are not the traditional stick-thin dancers.'" Another, 22-year-old Emma Roby, told the film crew she was worried about the show's reception. "I keep thinking of that sketch with Dawn French and Darcy Bussell on The Vicar of Dibley – what if everyone is laughing at us?"

Sleep is unrepentant, insisting they will have to get used to people judging them. "I wouldn't have taken this on if I thought they wanted to make a comedy out of it," he told me. "I want to be able to walk back into the Royal Albert Hall with my head held high. There are people who will take the mickey, but we are taking it very seriously."

Sleep likes to tell his outsized troupe that he too became a dancer against the odds and is, at 5ft 2in, still the smallest dancer ever to be accepted into the Royal Ballet School. He says he wants to show you don't have to be a "stick insect" to be beautiful. "Think of Cézanne, Rodin – their women were bigger then." He is adamant Big Ballet is not a well-disguised diet show: "We are not trying to make them lose weight."

Monica Loughman agrees: "You can be big and still be elegant. Back in the day, Marilyn Monroe was a size 14. Some of these women have amazing figures. One has the longest arms I've ever seen. She's just got boobs and a bum, too."

The show's executive producer, Emma Loach, danced with the English National Ballet School. She says no one wanted to put on a freak show. "Channel 4 was clear from the start they had no interest in making fun of people with bigger bodies than size eight."

Loughman is far more worried about the dancers' fitness than how they look. "We've been plagued by injury. One girl tore a ligament in the very first rehearsal," she says. "Fitness was my main concern – waking up their muscles and joints. But psychological factors needed overcoming too. Some of these women couldn't even look in a mirror. They couldn't accept the way they looked, which I thought was mad."

As we talk, one of the dancers is standing on a chair in her underwear, arms out like a scarecrow as she is fitted for her swan costume. Her cheeks redden as a camerawoman pans up her size 20-body, while a designer with a mouth full of pins nips and tucks at the material. Early on, Sleep and Loughman decided to spare the women the indignity of wearing tutus, and instead chose a 20s theme, with feather headdresses and flapper dresses. One of the women, chosen to play Odile, the evil Black Swan, is disappointed. "I've always wanted to wear a tutu," she harumphs.

The contrast between the dancers and the willowy Loughman could hardly be greater. How can you possibly understand how they feel, I ask Loughman, who is a size six. Her answer is robust: "I understand 100%. I have watched my sister and my mother struggle with their weight, but this is the way I'm meant to be. This is my size and I'm not going to apologise for it."

She says it's nonsense to suggest most ballerinas have eating disorders: "You can't be a ballet dancer and not eat. If you don't eat, you can't dance. There are dancers with eating disorders but they are the ones who don't get their contracts renewed." Indeed one of the Big Ballet stars, 52-year-old zumba instructor Christine Longster, won a place at the Royal Ballet as a teenager, only to be sent home after she "stopped eating".

Loughman says she and Sleep choreographed a 30-minute version of Swan Lake to "enhance the beauty of these everyday women – the choreography is very respectful, mindful and considered". She is adamant her show will be a far cry from Fat Ballet, a company of plus-sized dancers from the Siberian city of Perm, which she derides as "a pointless provocation. It is quite shameful, not because of their size but because of how vulgar they are."

Big Ballet is aiming for something very different, she says. "I want them to come away having had the best time – and hopefully having put some of their demons to rest."

Big Ballet is on Channel 4 on Thursday at 9pm.