Food labels would have larger-font calorie counts and more realistic portion sizes under an Obama administration proposal to significantly revise nutrition labels for the first time in two decades.

The proposal, to be unveiled by first lady Michelle Obama at the White House on Thursday, aims to make it easier for consumers to decipher unhealthy ingredients in packaged foods. The new label would include a line for "added sugars" to distinguish from sugars that appear naturally in food. It would list nutritional values...