A bigger cylinder holds more air, obviously, but is heavier and less streamlined. It's not unheard of for a diver to switch from a big to a small cylinder and find they both last the same amount of time as the decrease in air available is countered by the decrease in effort it takes to lug it about. 10, 12 and 15 litre are the most common single sizes.
Cylinders are made of either steel or aluminium. Steel is more negatively buoyant, so the general rule is wetsuit = aluminium, drysuit = steel. If you're using aluminium with a heavy weight belt, you may want to consider replacing it with steel so you can shed some lead. If you're using steel and no lead, you may want to consider aluminium so you aren't overweighted.
It's important to remember that aluminium cylinders, whilst lighter in water, are heavier out of it - Alu cylinders have thicker walls, so displace more water. Do not assume that heavier cylinders means more negatively buoyant cylinders. Fortunately, cylinder buoyancy characteristics can be calculated well enough for you to work out what you're getting.
232 or 300 bar are your main choices - it's very rare to get a true 300 bar fill, and it's not as easy to get a Nitrox fill to 300 bar. 232 is the most common pressure. Get a DIN valve, to go with your DIN regs, but make sure they're A-clamp convertible (most A-clamp cylinder valves made today are DIN with an insert) in case you go to a filling station with no DIN filling capabilities.
Redundancy is becoming a bigger and bigger concern. Some people cope by adding a small pony cylinder to their existing setup, typically a 3 litre. However, twinsets are considered a better system. The main concerns are that they are more expensive, too big, or too heavy.
The expense issue is true, the others are not. Twin sevens weigh about the same as a single twelve. A single fifteen plus 3l pony weighs about the same as twin tens. The lightest way of getting redundancy is a twinset.
I use: Twin 7s, Faber, OMS steel bands, SOS manifold, 232 bar.
SOS manifolds tend to be hard to turn when fully pressurised - a good dive shop can fix this. Runneymede Dive sorted mine out a treat when they O2 cleaned them.