Re: What's a good dimension of a flat for a "one size" option?
bigger than 27x27 AFTER washing.
small flats start at 27x27, then usually shrink to 24x25 (GMD smalls are one of the few exceptions). large flats start 32x32-ish, then shrink to hopefully no smaller than 27 etc.
btw, LOVE my diaper rite flats, 28x30 or so, great for pad folding and easily kitefold/jelly rolled to fit my DD at 10 lbs.