collecter header size?

can anyone tell me how to find out the collector size of a header before buying it. I was looking at a greddy, dc sports, edelbrock

needed to find out if any of them have a 2.5 in collector

Majority of them will not have the 2.5" collector. I'd just start calling retailers and go from there. But, you can always pick up a used one and then cut/weld in a 2.5" pipe.