Just as a forewarning, Sai is meant for illustration although it can be used for animation! I think it’s a good starting program to practice, but when you have the experience (and money), I recommend Toonbom or TV Paint, as they’re legit animation programs and are easier to work with.

Also, use gifmaker.me to make gifs! It’s free and fast. You can resize gifs there, too.

Have fun, and good luck!