The scale of your model is determined by the size of the doorways it must get through and the stairs it must climb.
Scale of your model has little affect on speed. Scaling does not affect face count and face count is what is important.
Block textures should be as small as possible until it affects the quality of the image.
Level scale is determined by the level design. To small and the the engine can't handle the angles. To large and you exceed the quant limit. So if your level consists of one three bedroom house, scale can be whatever you want. If you want 500 3 bedroom houses the scale will be very small.
So.. level design determines level scale--level scale determines doorway scale--doorway scale determines model scale.
*** use to be that the proper level scale for a shooter was 1 quant = 1 inch***
And of course if you build your entire level out of models that changes everything.