While different brands may vary on the exact specifications, generally speaking, the four classes of ukulele can be identified as follows:

Soprano: full length of 21 inches, scale length of 13 inches, and between 12-15 frets.
Concert: full length of 23 inches, scale length of 15 inches, and between 15-20 frets.
Tenor: full length of 26 inches, scale length of 17 inches, and between 15-20 frets.
Baritone: full length of 30 inches, scale length of 19 inches, and at least 19 frets.