The stand-over just over 19 inches for the smallest size. I can think of a few people on here who are looking for a 25 inch or shorter standover, but not sure what TT they need.
If their smallest frame has a standover of 19 inches, I'm wondering if it has full size wheels? Or maybe the 504mm standover is a typo. I have a Terry mixte in a 16" frame with a 700C rear and 24" front wheel and its standover is 23.4 inches.