Postcard, which is 100x150mm paper, is great for small personal prints that you may wish to send anywhere in the world via the postal systems. Plus Ilford postcard stock, is on a thicker than normal base.
5"x7" paper is great for a brag book that you can carry around, plus it is about the minimum size you can hang on a wall and still be looked at without having to stand too close to.
8"x10" paper is a staple format, full frame 35mm stuff centred with a smaller gap top and bottom than the sides, actually looks quite nice in a frame. I've been using this for a little while for my personal stuff at the suggestion of the missus and it works.
A4 format is really the better of the smaller paper sizes for hanging on a wall. The proportions very closely match 35mm and A4 frames are everywhere, so the choice is great.
After that I use 12"x16" paper which doesn't really match the neg proportions, but looks great from a distance.
These are the sizes I use for my own work to hang up in my own place. If I was to suggest a minimal outlay situation with the greatest possibility, it would be 8"x10" paper.
Apart from what I have already mentioned with this paper size, if you split it in half and print to 5"x8" then you get two prints that are virtually full frame proportionally. This size is the most popular print size with my extended family and friends.
A Jobo Varioformat easel is designed to allow you to do wonderful croppings with various paper sizes, with 8"x10" paper and this easel, you have about the most versatile printing system around.
There are other variable format easels, but I have never seen one as good as the Jobo Varioformat for simplicity and ease of use.
Mick.