A pace is slightly over 0.9 meters. Thus, a cubic pace is about 3/4 of a cubic meter. In Ars Magica, at least 
I think the basic idea of the authors was to simply rename a "yard" (a unit of measure familiar to them) to something that sounded appropriately medieval. On the other hand, you might try a slightly different tack on the term, and adopt the roman (double)pace, about 1.6 meters. This suits better the roman traditions of the Order, and makes it easier to use: 1 pace becomes the height of the average person, and it takes a nice, round 1000 paces to make a mile (which, after all, is where the name "mile" comes from -- literally "a thousand [paces]"). That's what we use in our games, and as long as you use it consistently, it causes very little trouble.