I agree -- try them on at the store.
My wife uses a set of mediums for her 8-1/2 pack boots. My recollection is that a size small stablicer will work with size 8 pack boot, but we found a the same boot 1/2 size larger wasn't compatible, owing to strap lengths.
I borrow the same set for my size 9-1/2 summer boots, or pair of large ones for my winter plastic boots (size 10). The Large size is abit too long for my winter boots, but the medium's don't have sufficient strap play - they just barely hold on, and often don't. Not the best thing to have let go...
On my last winter trip, the night before I was tired and hurried through my checklist. I managed to slip one left medium and one right large stablicer into my pack. Imagine my surprise the next day on the trail!
-- LTH