Your chain length is determined by the number of teeth from both the chain ring and the cassette (chain wrap). The cages on dérailleurs are different lengths (producing a different parallelogram or something or other) as they are critical for taking up the chain slack/tension as you move from one combo of gears to another.

The rear dérailleur cage for a large cassette is longer than that needed for a corn cob (a small, tightly spaced road cassette).

A front dérailleur also has a limited capacity for the number of teeth over which it will shift. The cage will actually be deeper for a triple crank.

Sheldon Brown probably has an entire epic dissertation on this subject -- especially now that compact cranks are the vogue and creating a whole new set of shifting challenges.