Steel drums can be of any size. The diameter is the same as a
55-gallon drum, but the height can be different. A steel drum
band can consist of varying size drums playing different parts.
The lowest part of a steel band is the bass. This part has 6 full sized 55-gallon drums with three notes on each drum. This commonly plays the bass line as with a bass in any other band.
Cellos would be the next largest pans. There are usually 3 drums making up the Cellos. The drums look similar to the bass pans, but the bottom of the drum is shorter, thus the pitch is higher.
Guitars are higher than cellos. Both the guitars and cellos share the same pattern of notes spread out over the same 3 drums. However, the guitar is much shorter than the cello. These pans sometimes play melody and sometimes play bass lines.
There are two drums that make up the double tenors and the double seconds. These parts can play melody and counter melody lines. They wouldn't necessarily be in the bass line, but could play background parts.
There is only one lead pan. The lead player plays the melody over top of the rest of the band. There could be instances where someone could write background parts while another drum is soloing, but for the most part the lead plays melody and solos. The pattern of notes for this pan follows closely to the circle of fifths.
A steel band can consist of the above drums, but doesn't have to. A small combo can use the lead pan, double seconds, a drum set, and an electric bass. This works better if there are not enough people to cover a full band. If a band has too many members there can always be found auxiliary percussion parts.