I use 15x25 for the 7x17, and I think it works very well proportionally.
However, for the 5x12 images I made last year, I matted them in 13x19 matts and I think they look very good. That is a standard format so sleeves are available, and the slightly larger short dimension helps give the smaller 5x12 images a little more heft.
Let me say, however, that I overmat the print but I don't cut the window down onto the image. The window is cut about 1/2" larger in both dimensions so that the entire frame and a black border is visible all the way around the image area. That makes the matt a little thinner. If I were cropping onto the image with the matte, I'd probably prefer a slightly smaller border width for each size.