I mat my panoramics (from my xpan) with a 6 1/2" x 18" window in a 12"x24" frame. This overmats a little, just covering the border of my filed-out carrier, more or less tight to the image area. If I want a black border on the image, I'd make it more like 6 3/4 x 18 1/4. The frame size would stay the same.

I did one where I had a spare 20x24 frame sitting around, so I put the 6 1/2 x 18 in the middle, with a slight topward weight (it is a horizontal image, so the huge bottom and top borders don't look too ridiculous). I wouldn't do that again, just because it wastes too much mat board, but it looks nice.