Measuring for dressage cinch? The measurement I took seems a bit long...and... where do I buy a LARGE girth?

If your dressage saddle has short billets like a hunt saddle, then you would use a regular hunt type girth. Dressage girths are made for long billets, which is why they come in such small sizes. And yes, they are measured buckle to buckle just like any other girth. Most dressage saddles use the long billets/short girth system, which is why dressage girths are short. But some dressage saddles like yours are made for regular girths. It's not better or worse, just different.