Small as possible tends to look right IMHO. The problem often is that the sides tend to look better at a thickness smaller than the smallest hinges.
My rule of thumb is to make a jewellery box 300 x 200 x 100mm with about 15mm sides. The last butt hinges I bought were from Isaac Lord and were about 20mm x 10mm. I seem to remember that they were not on the website but it was 12 months ago. They were solid brass.
I sometimes change the box size a bit, but stay with roughly 3x2x1 as a ratio, or 2x1x1 or square. I tend to start at 15mm for sizes and maybe drop a mm or 2 if the box is smaller. The next ones that I make I am going to try some lift off lids.
These dimensions work for me, aesthetically. I know that other people use different ones.