As the question comes up occasionally it is worth noting that I play almost everything in 15mm and the game rules usually just assume that. There are a few exceptions such as Laserstorm which targeted 6mm but the baseline for me has always been 15.

It turns out that the weapon ranges and movement distances I like in 15mm also happen to match up pretty well with what is the norm in 28mm so there is rarely a need to modify distances to fit another scale. For example a 4-6" move feels pretty good in 15mm and thats about what figures tend to move in games written for the larger scales. 

The area where this can get a little awkward is when it comes to interacting with terrain. A 4" climb looks pretty casual for a 32mm space marine but is a pretty epic endeavor for a 6mm trooper! I tend to not worry too much about things like that, but if it is a worry you should probably tweak those specific distances (halving for 6mm works fine).

On that note if you do want to scale down the game (particularly for playing on a little table) I used to suggest swapping the inches for centimeters (so 6" becomes 6 cm.) but I think that can get too fiddly. Instead these days I would suggest just halving everything. A 6" move becomes 3" and a 24" weapon range becomes 12". Try it either way and see how you get on.

The latter is also useful if you want to play on floor tiles or a grid and count squares. Most grid surfaces don't give you very many squares across so 1"=1 square is often going to make things move too fast. 2" = 1 square requires a bit of rounding up or down but tends to work better in my experience.