It has been suggested that I have a collecting addiction
but I do have one rule I stick to, which is 28mm scale.
Ok - that does include old 25mm and lately some stuff labelled 32mm but which seems to fit.
I am guessing 28mm is supposed to be to the model's 'eye level' whereas 32mm is to the top of the head.
1:56 is the guide.
This 'rule' used to be accompanied by nothing past year 1800, but that has been broken in the last couple of years - modestly, but still broken.
The Golden Rule is to buy with versatility in mind.
Figures should be usable in units for large battles or in smaller combinations for skirmish games. Having started with Warhammer Fantasy exclusively for many years, disillusionment with GW when the "Goldswords" box was released got me looking around, and these days we really are spoilt for choice.
I started to do an inventory of my Rules Sets Books, and discovered I have another addiction.
Even leaving out expansions I have some 35 different ones, and as Zero23 suggested, spearmen are spearmen in lots of situations, bowmen appear across several centuries, halberdiers or billmen or polearms are common soldiers through multiple 'ages'.