The number refers to the size of the bore. 3/4 tuba is the smallest number, and the smallest tuba. It's just plain and simple not very big. The tubing of a 4/4 tuba is just a little bit bigger around - larger diameter, especially nearing the throat. Nice is the normal tuba. 5/4 is a larger than normal tuba - the tubing is even fatter. a 6/4 tuba is just simply huge. I've heard them jokingly referred to as B.A.T.s, or Big Ass Tubas. A 4/4 tuba can be taller and more impressive looking than a 5/4 tuba, but what matters is how fat the tubing is. Read this article and observe the pictures:
http://www.rickdenney.com/tubas_compared.htm
For the keys... BBb is the lowest we use. It's the contrabass tuba in the USA. In the UK they may use a CC tuba instead, though it still has its few uses in America as well. The Eb tuba (USA) and the F tuba (UK) are the bass tubas, and are higher in pitch. They are ideal for higher music. Ever seen Holst's Suite in Eb? Tuba part goes pretty high, can be nasty on a fat BBb. In the USA, we just need to learn new fingerings. If you get an Eb tuba, your Eb is an open note, as well as the Bb above that, than the next Eb, than high G, than high Bb, and on up. So you need to reprogram yourself with each different tuba you pick up as to what notes are going to be open notes and how to finger the rest. It's not that hard if you put your mind to it.
