The Concert has a scale length of between 36 and 41 cm (14 - 16 inches) it is usually about 38cm (15 in). Why this is referred to generally as a Concert size and not an Alto when all of the others go with the musical sizes I don't know for sure?. Whatever the reason it came about in the 1920's because Martin was finding that the Taropatch it had in its catalogue was disliked because of the difficulty in keeping 8 strings in tune, but liked because its larger size gave it more acoustic volume so they started to sell the 4 stringed version. This original still have the 12 frets to the body but these days they often have a longer fret board, meeting the body at the 14th fret and going further down the body than the Soprano
Like the Soprano there is a sub set which have a Concert size body but a Tenor neck to accommodate more frets, called Long Neck or Super Concerts, sometimes even Mini Tenors, however even fewer manufacturers make them than make a Long Neck Soprano.
Here are all my Concert Ukuleles
Click on the Ukulele for more information about it