The ukulele comes in many different shapes and sizes. This significantly affects the tone they produce and the style of music you’d use them for. A soprano, for example, is the typical ‘small’ ukulele you’ll see in classrooms. These usually have 12-15 frets and run around 21” long, and are perfect for young players or people who want a very compact instrument.
Above that, in size order, you have a concert uke. These are arguably the most popular and are typically what most people think of when they picture a ukulele. So if you are in doubt about which to choose, you’re pretty safe with the concert size. Next up is the Tenor uke, which usually comes in around 26 inches - versus the concert's 23 inches. These tend to be more comfortable for taller players, so if you feel like the concert is too cramped, this is the way to go.
Lastly, we have the baritone. These are usually around 30 inches and produces a low, deep tone, perfect for playing with other, smaller ukuleles. However, it’s also worth noting that the baritone is tuned slightly differently to the other “standard” ukes - D/G/B/E - and therefore can be tricky to play at first if you are used to the regular ukulele chord shapes.