Road bike recommendations

The tyres limitation is more about which tyres are readily avaliable from a typical bicycle shop, those consideration I usually take into when you're building a commuter.

650c can be found in some shop, but you'd need to do some hunting, there may be 650b mountain bikes tyres as we speak but for road tyres, even less likely.

Hence 26" being easier to find in almost every shop, especially since it's still fairly common for people to convert their MTB with slicker tyres for commuting.

Does she really want to have wide tyres? In designing a frame for 650b and big wide tyres with noppies one is going to get pretty darn close to designing a frame for 700c with usual tight clearances.

It's still possible to build a small frame without toe overlap, 650B with 40mm still come out smaller than 700c with 23mm;

1 Attachment