For my cardboard box bass I went with the 3/4 size. There are a few sites for getting dimensions. Also weed wacker wire is excellent for making strings. If you have any questions send me a message and I'd be happy to help with the construction layout. So far it's been working really well!
Shooting for the 40" range is probably good. Keep in mind that you never see a full size bass. Most people play 3/4 size and if you're really brave, you play a 7/8 size. (Like BenBob said, upright strings may be a bit prohibitive. You may want to look at weedwacker if you're not planning to bow it).
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For my cardboard box bass I went with the 3/4 size. There are a few sites for getting dimensions. Also weed wacker wire is excellent for making strings. If you have any questions send me a message and I'd be happy to help with the construction layout. So far it's been working really well!