It’s a fair question. I’m trying to build this
https://www.etsy.com/listing/892641...allery-1-3&from_market_listing_grid_organic=1 (well really this
https://www.youtube.com/watch?v=f-i49gCjNbQ but it’s a video so hard to see)
But without knowing specifics regarding string type, scale length et al. They tend to be very variable and reportedly forgiving to make but I would at least like to gather what info I can ahead of time.
Because of the physics of strings there is a range of tensions at which it works nicely, roughly 5 semitones or so. Below that it sounds muddy and won't intonate well, above that it might snap.
You can use this to estimate starting gauges for your strings by using your knowledge of other instruments. The instrument in the video looks to have around a 17 inch scale, which is the scale of a tenor uke. If that's what you decide to build, work out what notes each string is tuned to and find the closest equivalents (within 2 or 3 semitones) from a tenor set. Use those strings for your first set.
Once you've strung it up and played it you can decide if each string is about right, too floppy or too taut, and then go up or down a gauge for that string on your next set.
Here's a worked out example for my baritone uke which I wanted to tune in 5ths, GDAE. I reckon its 19 inch scale is the equivalent of a classical guitar capoed at the 4th fret, so my choices were:
G - guitar 6th (E, up 3 semitones)
D - guitar 4th (A, up 5 semitones)
A - guitar 3rd (G, up 2 semitones)
E - Guitar 1st (E, no change)
In theory the D should be a little too taut, but it's not too bad. Still, when I re-string I'll use a string one gauge thinner than the guitar A.
The E plays better than I expected, but I could do with a string one gauge thicker next time.
If I'd wanted to get the tensions more even across the strings then I could have bought two sets of cheap classical guitar strings, one light, one heavy, and used the best strings from each set according to my mental calculations (which would be, the light set=the heavy set tuned down one fret).