I use 0.25 for the scale for most everything. The exception is when a solid color is used where I use outrageous scales like 64 as the quality isn't affected no matter what it gets set to. Anything finer than this has little meaning. This means that, if you have a square occupying a 16x16-quant area and you UV map it so it takes a 64x64-pixel area, you'll get the 0.25 scale (as from WED blocks for comparison).
The maximum level size, with model-only levels, is a span of a little over 4.3 million quants, but with tricks, it can be much larger.