Which Size Ukulele?
There are four main sizes of ukulele. From smallest to largest, they are:
What's the best size ukulele?
It depends what you want to do with it. If you like to play a lot of solo pieces, a tenor ukulele will give you more room to play and more volume.
If you're buying your first ukulele, the soprano is the best place to start.
Before too long, you'll probably have one of each.
How is the size determined?
The size of the ukulele is not determined by how big it is, as such. It depends on the length of the vibrating part of the string - between the bridge and the net - also known as the scale length.
Soprano - 14 inches (and under)
Concert - 14 1/2 to 16 inches
Tenor - 16 to 18 inches
Baritone - 18 inches (and over)
These aren't set in stone. Some companies, just to confuse things further, classify their ukes differently.
There are also some other, much less common sizes: