I'm surprised that no one mentioned the test strip printer that is showed in Way Beyond Monochrome editions 1 and 2. This is a fantastic way to save paper! I made one years ago and love it. Its not suitable for every negative but I find it useful for most.
Regarding using smaller size paper to make test strips for larger sizes I just don't trust it. If I'm printing 11x14 I will take a piece of 11x14 from that batch I'm going to make the final print on and cut it up into smaller pieces to make tests. If you do the math the sq inches of paper is all equal so there is no point (economy wise) really in trying to use 5x7 paper for tests then use 11x14 or 16x20 to make the print. I have tried using 5x7 paper for tests then final print on 11x14 and sometimes this worked and sometimes it didn't. I think you almost have to purchase both size papers at the same time, or hope they were produced about the same time... etc.
So my recommendation is to cut up whatever size paper you're printing on for your final for the test strips. You can get alot of 5x7 pieces out of a 16x20 sheet!!