Don't worry too much. The mistakes will teach you far more than getting it right first time :-) The rule is: 'fit the biggest motor that will go in'. Next rule: 'DCC no flywheel - DC the biggest flywheel that will fit in.' Next rule: 'when you compromise on motor size stay with the largest diameter armature practical' Shorter isn't as much of a compromise. Next rule: 'forget coreless motors unless you're really really tight for space'. A high quality can motor does everything we need for a model loco. Next rule: 'don't buy cheap motors for locos'. Next: 'buy good quality gears and gearbox'.
Greg.P.