A bit awkward, the whole “size” discussion for a gift, let alone for a surprise… I think I can “work it”, but if I don’t come back, send out a search party…
Why? Because you’ll be tangled up in panties? It’s easy, slide them over your head then realize they can’t get past your boobies, then throw them out of the window. Problem solved.
Also, don’t rely on pics. It’s a sexy six pack not a case. 
Good point, except she’s a redhead, not a blonde - it won’t be that simple, and MUCH more fraught.
Just take a peek in her drawers (dresser drawers, ya pervs) and check the size label on others you have seen her wear recently.
(don’t dig to the back of the drawer, that where the “might wear this size later” undies live, LOL)