I like to either stick to 3:2 - so that's 7.5 x 5 for the family album or 12 x 8 for the real classics
Occasionally go square format 8 x 8 (though usually by trimming the 12 x 8 as that is usually cheaper than 8 x 8, doh!)
suspect I am a Hasselblad fan at heart as the square format makes so much more sense to me (technically and aesthetically) but no one makes a square format digital I can afford
4x6 for casual viewing. 5x7 and 8x10 for wall hanging. I have one 11x14 that was kind of special so I blew it up. I don't do any of the printing myself and find the 1 hr services just fine.
Of course, if were to go professional that might change. Not forseen in the near future.
Most of my pictures are printed in the 2:3 ratio - 6X9, 8X12, 10X15. I won't force to that ratio;if the picture seems right cropped square or some odd ratio, it gets printed that way. I usually do a final print on a piece of paper large enough to have a fair amount of white space around the print;if I plan on matting the print, then the margins will be smaller. I use Qimage for printing;size is currently limited by the max width (13") of my printer.
I currently use an Epson 2200 with Epson inks and mostly various Epson papers. At least half of my printing is B&W using QTR, with good results. As soon as I can reconcile my space constraints I will be getting an Epson 3800 (17") and will probably do some testing of other papers.
For the pics that will be going into my scrapbooks, I generally stick with 4x6. At the moment Dotphoto.com seems to have the best printing rates around--9¢ each, otherwise I'll go with Adorama, if they're having a sale. For larger prints, Adorama is my #1 choice, as their prices are reasonable (and they do have sales!), they ship quickly, and they offer a variety of different choices of paper that they will print on (the metallic paper is pretty cool).
I do most of my printing on an older Epson 1280 dye-ink printer using the Epson inksets on Epson Premium Luster or Ilford Galerie High Gloss or Smooth Gloss papers of Super B (13"x19") size. Depending on my matting, I crop to 11x14 or 12x18 sizes. Both look good on the wall and in my portfolios.
I print all my own on an old Epson Stylus Photo R300 using Kirkland Professional Glossy Inkjet paper available from Costco. I believe it is made by Ilford.
If getting printed out ... 6"x4" for family snaps done at home.
I get 8"x10" and 11"x 15" done at my local camera store (the owner is the one that should be the light of Pentax over C or N as well) ... I get looked after by him. I don't go anywhere else.
I use a Canon Selphy dye sub printer that can only handle 4x6" prints, but the image quality is excellent, no dots from sprayed ink and watching it print each CMY layer seperately is quite entertaining.