By the RAW, you are only allowed to EITHER move or attack, not both in the same round (see page 90 HARP Fantasy).
I used minis and battlemaps for most combats with a grid of 1" and defined it as 5' (~1 5.m). I house-ruled that the players were allowed to move up to 1 grid (1.5m) and were still allowed to use any other action without a penalty (e.g.: Melee, Missile, Spells). I never had any problems with that rule.
There was a PDF product called BattleMaster (from Mjolnir). It had ~5 pages of rules how to play HARP with Minis, however I never used them as I was happy with the above and it had NO changes of the length of a round.