If the pads are coming well around your hips and there is only and inch or so of adjustment left on each side when tight then it's a medium. If the pads just come around your hips when tight and there is a couple of inches of adjustment left on each side when tight it's a small.
bbrown and I recently went thru this. I bought a large belt off him when my "medium" did not wrap around. Between us we had all 3 sizes.
These are the end to end padding lengths he measured and I confirmed
Small=30"
Medium=32"
Large=40"
I bought my setup used. I have a 38" waist and was told I needed a Med by a guy on another site. I probably should have called Aron. The Med was obviously too small and the large fits but only has a little wiggle room.
We both came to the conclusion that they are either custom built or there should be a "Marge" sized belt around 34-36"(bbrown came up with that).
anyone know the measurements of each size for the omni belt? i have a spike camp with a belt that is too big (i ordered a small) i suspect it's a medium. had the same problem with my T2. belt was to big even though i supplied measurements. swapped it for a smaller belt and it measures smaller than my omni belt. it's a bit annoying having to trade belts to get the right size for a new expensive pack.