I need to replace the seatpost collar on my 2005 Trek which has a 27.2mm seatpost and I don't know what size collar to get. Do the collar sizes reflect on the outer diamater of the seatpost or the clamping area of the post instead?

For instance. I'm getting a Felt F3c frame which utilizes a 27.2mm seatpost also. Are the collars interchangable or are they frame specific?

Any advice would be appreciated. Thanks.