OK, the help Guide (from the HELP MENU internal to the software) is actually very helpful.

The thing is, ALL the effects are expressed in UNITS. How much of a BLUR EFFECT do you want? Type in your units (or fraction there-of). How much do you want to 'fade' an edge by? Again, based on Units. Same for BEVEL, GLOW etc.

So, because the majority of the maps I make are dungeon maps (do a search for Thunderspire Labyrinth, Pyramid of Shadows, and Trollhaunt Warrens), my maps are initially made where a Unit = 5 Feet. So, knowing that,I know that if I make a GLOW effect equal to .1 Unit that is equivalent to 1/10th of 5 Feet or 6".

Does that help?

Also, if you are trying 'blur' on level into another, you want to make use of the EDGE FADE Effect, not BLUR.