in the prefs, check what units your in, the defaults cm, then just make the object to the dimensions you want a 1x1x1 cube is therefore 1cm in X,Y,Z, but you can choose what dimensions you want cm, mm, m etc. But remember all calculations for dynamics are in cm if I remember correctly or is could be meters, no I think its cm.

If you want it really accurate, use a measuring tool under the create menu to set out sizes