I got curious about thickness and started taking some measurements to determine exactly how thick a diaper needs to be to achieve maximum realism, as to what a 3 year old baby today would be wearing.
Average 3 year old boy's leg length = 425mm
Modern Diaper Measurements, all numbers rounded up.
Folded thickness = 21mm, 5% of leg length
Folded length from top to crotch = 235mm, 55% of leg length
Folded Width = 121mm, 29% of leg length
That would translate to...
Average length of 21 year old male's leg = 900mm
Requiring a diaper that is...
Folded thickness = 45mm (1.8 inches) (7.2 inches in quad fold like a lot of diapers come)
Folded length from top to crotch = 495mm (19.5 inches)
Folded width = 261mm (10.3 inches)
It would also need to have a front unfolded width of 23.7 inches, with stretch to 41.2 inches, a rear unfolded width of 27.6 inches.
Also, 4 inch single tapes located 1 inch from the top of the diaper.