Well, I make and sell a lot prints. I try to confine things to 5x7, 8x10, 11x14, and 16x20, but just this past week I delivered several 26x20 canvases that got framed, and also had one client want everything in 4x6 because that is what her photo albums were. I also have to deliver images in both low and high resolution for a client every week after the Sunday polo match, so that things can go into electronic newsletters (low res) or be shipped off for magazine prints (high res), including posters that are 40"x60" or so.
So, it is important to be somewhat flexible in what is delivered, but at the same time, not redoing things for various sizes. I sort of settled on an 8.5"x11" size image at 300ppi in my RAW conversion and process. This lets me take things down or up fairly easily without having to reprocess. (This is really important if there are a number of retouches made, and folks want multiple copies at different sizes. Everything coming off the printer must look the same, be it the 5x7 glossy or the 20x26 canvas. There will be some slight differences due to formatting, but the main subject has to look the same across all copies.)
The one thing that I have done is to discourage odd sizes, like 4x6. So, I price things accordingly to drive business toward the image sizes I prefer to print, like 8x10s or 16x20s.
I also print most of my stuff on an Epson 7800 using the Colorbyte ImagePrint RIP, and that has saved me more time and money and provided truly matched prints to deliver to folks every time, regardless of the media for the most part. I also do most of my printing on Epson Premium Luster (now called Ultra Premium Photo Paper Luster) for sizes to 16x20. Bigger than than usually goes to canvas. I rarely print on matte finish, unless a client requests it. The gamut is lower, and it usually takes more ink to achieve the same look, and the blacks are never quite as deep, though some of the newer matte finish papers are getting there.
LJ