My first "build". Need some help.

I'm going to try to build a cigar box style ukulele based on an Eddy Finn ukulele I saw on the internet. I found a box I'm going to use for a body. I'm planning on a tenor in low G. The necks I'm looking at don't give actual dimensions but they say "for 26 inch ukulele", so I'm pretty sure thats what I need. I'm not sure about the fretboard though. Do I go by dimensions or by frets? I'm not sure what I need. The other thing is that I don't know how long my strings need to be. I assume the distance between the nut and saddle/bridge is pretty important.

The necks that say they are for a "26 inch ukulele" mean they are for "tenor" sized instruments which means a 17 inch scale (more or less). The spacing of your frets is determined by your scale length. There are many calculators on the internet that will give you the distance between the frets on the fretboard. If this is confusing, do some research on "scale length" and it will make sense.... Wouldn't worry about string length at this point. Build the thang first. Then buy the length you need. What you do need to think about is where that 17 inch scale lands on your box because this will dictate where your bridge and saddle will land and is important to how it sounds. You want it to land on the sweet spot. Not too far forward and not to far back. I don't build cigar box ukes, but I'll bet somebody has figured out the best spot.

A tenor uke has a nominal scale length of 17". That's the distance from the bridge saddle to the nut.

If you buy a ready made neck for a 26" or 27" instrument, that is for a tenor. There will be no wood at the heel for a joint: it will simple butt onto the box (at the 14th fret). A dowel, biscuit, loose tennon or bolt could be used to reinforce the joint.

Similarly if you buy a ready made fretboard for a tenor sized uke it will be nominally for a 17" scale length. It may have 17, 18 or 19 frets-that's not really important-most of us dont play at the end of the fretboard.

The distance from the nut end to the centre of the 12th fret on your fretboard should now be measured. It should be nominally 8.5" (half the scale length). It may be a little more or a little less than this. This measurement should be doubled for you to determine the actual scale length of your instrument and therefore the bridge/saddle position. (For example, if its 8.6" then your instrument must have a scale length of 2x 8.6 = 17.2")

When you put your box and neck/fretboard together and measure to find the bridge/saddle position, it should ideally be midway between the end of the box and the soundhole. If the box is too small, the bridge will be almost at the end of the box: if the box is too big the bridge will be almost at the soundhole. You wont get the best sound out of the instrument if the bridge isn't ideally situated on the box. You need the right sized box!

There's one final point. If the actual scale length based on your fretboard is 17.2, it's necessary to correct or compensate the distance from nut to saddle to get perfect tuning. This means in practice that we add approx 0.15" to the scale length (in this example 17.2 +0.15 = 17.35") .

Just buy a set of tenor strings with a low G-they will be the correct length.

Ok, that helps me understand a bit better, thanks guys. I still have a couple questions though. My Kala KA-C has 19 frets and the fretboard meets the body at the 14th. 15 inches nut to saddle. Does that mean in theory if I moved the saddle 2 inches it would be a tenor?
The box I'm going to use for the box uke's body is just a hair longer than my Kala and the height and width (at it's widest point) are about the same. The box has more volume and surface area though. But if they are about the same length does that mean I'm basically putting a tenor neck on a concert body?
Im not going to do a traditional forward/center sound hole. I'm going to do 2-3 small sound holes. So I won't need to worry about the bridge being to close.

Ok, here's my rough plan, let me know if it's correct.
Attach the tenor neck to the box, glue fretboard to neck, measure 17.2 inches from nut to get saddle placement, attach the bridge/saddle, install tuning machines then string and tune with tenor strings.
That will get me a uke that uses the same chord shapes and positions (gCEA) as my Kala, right?

Well, no. Never mind the number of frets on your concert, or at what fret the neck meets the body. If you move the bridge you'd need a new fretboard.

The only fret you want to measure is the 12th. By that I mean from the face of the nut to the middle of the 12th fret (assuming the strings last contact with the nut is at its face, and that the middle of the fret is the highest point, but for all intents and purposes skip this parenthesis).

The distance from the nut to the 12th fret is half your scale length. Double it and you'll know where the saddle should be, and place your bridge accordingly. But before you do that read something about compensation. There is plenty of info on that topic right here on tjis forum, and I don't think you should whip out any tools before you know more.

There are always things to learn but some of them must be learned before you start.

Good luck / Sven

The plans on cigar box nation
http://www.cigarboxnation.com/page/free-plans
Dan Hulberts page
https://circuitsandstrings.wordpress.com/free-plans/
And instructables
https://www.instructables.com/howto/Ukulele

Should make useful reading.

Thanks guys. I'm getting a much better idea of what I need to do. It was hard to find resources that could answer my questions when I didn't really know​ what to look for.
I was originally thinking about getting a piezo pickup and a preamp but I think for my first attempt I need to keep it as simple as possible. (Which is why I'm going with a box uke rather than building a body). I want to make sure I can figure out the basics first. This first box uke is pretty much just a prototype and a personal test to see if I have the ability and patience. If this first one goes ok I'll get a little more advanced and start putting more concern in tone woods and other details. I don't think I'm going to worry too much about the finish either. The neck and fretboard I ordered come pre-finished, I'll probably just use a few light coats of what ever stain and/or oil I have laying around the house​ from previous projects.