I used the sizing guide at the manufacturer's website which says:
XXXS:* * Rabbit 0,6 - 0,8 kg
XXS:** * Rabbit 0,8 - 1,5 kg
XS:** * Rabbit 1,5 - 2,0 kg
S:** * Rabbit 2,0 - 3,5 kg
M:* * Rabbit 3,5 - 4,5 kg
L:* * Rabbit 4,5 - 5,5 kg
They're quite stretchy due to the Lycra content which seems to be what makes them fairly immune to bunny teeth. Sugar wore hers for two weeks and while it was looking a little tatty by the end she'd not managed a single escape from it.
If, heaven forbid, I need to put a bun through a spay again I'll be getting one of these ahead of time and delivering it and the bun together the the vet.