The last time I really checked what it does (albeit a while ago, so it may have changed), a higher biome scale differentiates Earth biomes into bands of desert, forest, and snow to a greater degree than a lower biome scale. I don't think it does anything to other biomes.
Actually nope, I just went and tested it. While what I posted may have been true in alpha or earlier beta, biome scale seems to have no effect at the moment.
i imagine it is for spreading or scattering the various teraintypes and objects so for example if you have a small lavaplanet with a low magmahight level that it isnt just a single big sea on your planet but maybe more smaller ones or changing the shape of the sea more to a river
