The usual terms of reference used are 1 unit = 1 metric metre.
You also may need to factor in practicalities like default 3d sound parameters, default game texture UV coords as set by modelling programs, etc, which often use a metre as the unit value. You can change these, of course, but it makes your life simpler if you don't have to.
You also may need to factor in practicalities like default 3d sound parameters, default game texture UV coords as set by modelling programs, etc, which often use a metre as the unit value. You can change these, of course, but it makes your life simpler if you don't have to.