If they get new stock to replace the existing 170s, whatever they pick should be the maximum length that fits in the platform in Stansted. if that is a 5-car, get a fleet of 5-car FLIRTs I would say.
However, people do often say that FLIRTs have short vehicles, but I'm not entirely convinced by this, as they are pretty close to the standard 20m carriage, which is the shorter standard, but still very much a standard.* A 3-car 755 is 65.0m long. While that is shorter than a 3-car 170 (71m), it is quite a bit longer than a 2-car (47.4m).
*obviously, there are now effectively 4 standard coach lengths in the UK. ~20m, ~23m, ~24m, and ~26m.