I'm happy using my metal ones with the wooden bottoms that I bought from Bass Equipment (bassequipment.com).
When I made mine, I used the plans from my 4-H book, and nailed plywood together on a frame of 2x2s. If I remember correctly, it was 12" wide, 18" deep, with a 4" board at the front that angled back to about halfway where it had a ceiling. It was 12" tall from that point back. Those measurements are as measured on the outside. It was pretty big, so I should've made them smaller.
Nestboxes should really only be just barely big enough for the doe to fit in to feed, but not get comfortable in there. Does will trample kits when they stay with them too much.
I would suggest looking at Bass Equipment for the measurement you need for your breed (they give the measurements for each of their 4 standard nest boxes, and the breeds that use each), and use that as your size guideline.
I like wood because it's heavy enough that the doe will leave it in the corner you put it in, and it won't hurt her if she decides to chew on it. The only down side is that it's not as easy to clean/sanitize.