The size (volume) of the composter is an important factor, but not the only one. The processing rate can be speeded up by increasing aeration (including thorough mixing or batching) and raising the temperature.

A composter is not just a storage tank, so buying a larger composter does not mean you will get the same efficiency of processing that you will get in a smaller one. The larger the chamber, the more important it is to get air to the composting material and avoid compaction. You might even consider installing two smaller ones instead of one larger one.

Large volume does have advantages: Larger systems serve more users in a fixed time period. They allow longer retention and processing time (for a fixed number of users), which might produce a more thoroughly processed end product. Larger means fewer maintenance events too.