The corona is an old standard size. In traditional size charts it is longer than a robusto and has a thinner ring gauge, and in many markets it used to be the standard cigar shape. Actual factory measurements vary. The corona, corona gorda, and petit corona are related sizes, but they are not the same.
A thinner ring gauge means more wrapper in each puff and less room for a thick core of ligero. This is about construction; it does not mean that coronas are always mild. Both Cuban and New World companies still make this size.
Do not confuse the corona size with brand names that use the word “Corona” on some bands. One is a size; the other is a trademark.
Full guide: corona and petit corona. Sizes: ring gauge and length cheat-sheet.