I wrote a post a little over a year ago about how I make underwear covers. I had to make some more because Darth Monkey has actually burst some of the seams from the last batch. Both Darth Monkey and the bear cub are wearing 3Ts right now and they were in around 18 months when I made the last batch.
For these covers, the rectangle I started with was 13 inches x 20 inches and I left the sides at 6 inches and the part between the legs 5 inches. The bear cub's waist measures 19 1/2 inches and Darth Monkey's is 21 inches. For the waist elastic, I used 18 inches and 10 inches for the leg openings. These should fit the girls for a while - at least another 1 1/2 years, I think. So you could use these measurements for any child in a 3T, but you might have to increase the lengths of elastic used.
These get used around the clock here and for Darth Monkey, because she is on a daily antibiotic still, we often go through more than one a day, so even though I made 22 in total this time, I might still end up making more in the near future, but I'm hoping these will hold us since I can still use the older ones that we still have on the bear cub.