Nothing, they are the same thing. The original ukulele was what is now referred to as the soprano. The larger concert size was made to get a bit more sound for a bigger audience. And then the tenor size, bigger, deeper sound.
The picture on his web site shows a ukulele that does not have a logo on the head. It doesn't have a recognizable shape that might identify it. It may very well be a custom made ukulele. I think it is a tenor size, but it could be a concert size.
It will depend partially on the type of sound one is looking for. Train uses a Tenor sized ukulele, but a concert size would also be a good fit.
The ukulele he played on American Idol was an Oscar Schmidt OU5 which is concert sized. The OU5 features a Hawaiian Koa top, back & sides and has an Abalone inlay, Rosewood fingerboard and bridge and Grover tuners.
Ukulele strings for the appropriate sized ukulele. Sets of strings are made by companies such as Aquilla, Martin, Gibson, D'Addario and others with the correct sizing.
Certainly. There are many ukulele groups and solo performers that play the ukulele. I've seen a sold out concert with over 2,200 people to watch Jake Shimabukuro.
It will depend on what sized ukulele and what size guitar as well as how they are tuned. A 'standard' sized ukulele is going to have a higher pitch than most guitars. However, a baritone ukulele is going to be pretty similar to that of a guitar.
LU-21 is the designation used by Lanikai Ukuleles for their standard size ukulele, sometimes referred to as a soprano. It is usually a pretty nice Nato laminate with geared tuners and a nice binding on the edges and a rosewood fret board. LU-21C is the Concert sized and LU-21T is the Tenor sized.
It varies depending on the type of ukulele. The Tenor has a wider range than a concert or a soprano.
Most of the musicians playing ukulele in a group are using a Tenor sized ukulele. Fenders are popular for this.
He used a number of different ukuleles in various performances. Most appear to be custom made. His main ukulele was a tenor sized, which looks pretty small in his hands!
About 24 inches.