Help! Birthday failure meltdown 😩

Okay I'll try be short and sweet.. My partners birthday is soon and one of the things I desperately want to get him are these Ryderwear trainers as he's a bodybuilder/personal trainer and they'd be great for him. They said their sizing is unique and I need to measure his foot.. But it's supposed to be a surprise?! They're a lot of money so I can't wing it as usually American sizes are one size bigger than uk ones, it's just the fact they've said their sizing is specific to their brand so how on earth do I measure his foot heal to toe without him knowing?! 😩😩😩