Concert Ukuleles (sometimes called Alto Ukuleles) are 23" long. The fretboard is about the same width but there is more spacing between the frets than a soprano. There are typically also more frets (between 15 to 18) than  the smaller Sopranos (usually having between 12 to 15 frets).. They have a distinctive sound closer to the tenors. Their small size also makes them great for traveling.