They do stretch a bit, really – tho' more around the instep (ratchet) fastening area. I picked up some off a forum member some years back who'd got 'em the wrong size – I still haven't needed to wear 'em yet, but trying 'em on recently they felt a *little* short – but they're almost exactly the same as my current pair, and next size up would eventually stretch too much for the ratchet to work I suspect. My very first pair of Sidis – early 90's – got to the point of feeling like a pair of slippers 😉
If they feel a bit short – rather then generally tight – make sure you've cut yer toenails 😉
If it's a bit tight across the toes you could try a shoe stretcher mebbe ?
I guess just get the thinnest socks you can find (silk?)
Or just wear 'em for a while barefoot and see how you get on – but I guess if you were thinking about returning/relling them you might not want to. They don't *seem* to suffer like trainers do for getting smelly, mebbe the lorica breathes somewhat better 'cos it's more leather-like (and I always use socks)- YMMV.