How to choose optimum length for op-amp design?
First of all, stop thinking of transistors in terms of resistance, think in terms of transconductance, understand it, use it.

First order effects:
- current density, regime of operation, range of operation.
Your W/L ratio and the bias current are the really the only things that you can change in a given topology.  The characteristics of the devices' in sub-threshold, Triode and active region are determined by current density and length.  Typically the MOSFETs in the diff pair will be active, short and high transconductance.  Your bias generator will be long and operational in the triode or subthreshold.

second order effects:
- Matching, noise etc.
General statements about matching should be avoided, it is process dependant.  For example pocket (Halo implanted) transistors are designed to match at Vth.  More Analog like transistors have different Vth vs. L relationships and also tend to be different.  As an example, one mechanism for mismatch is LER (Line edge roughness) of the gate, to control this you will need a wider gate, but to maintain the same W/L you then need a longer gate as well.  In this case a longer gate appears to match better, in reality it is because it is wider.  But is is one of many effects, and some times a longer gate DOES match better. Typically matching is accomplished through layout not just length.

In certain types of noise a larger gates is noisier and in others a smaller gate is noisier.  It is often better to run more current through, make a shorter device with more Gm and then filter out the out of band noise.

In short there is no one simple answer, and no substitution for learning.