you can just create the object without drawing it out...then you can input the scale you want. it'll automatically creat it as 1x1x1 (whatever scale you have set, in, m, mm, etc), then you can adjust the channel box to whatever you want it to be, and it'll scale within those units.

The reason it's 1x1x1 whatever you create is probably because you're "drawing" the box instead of just telling maya to create the default.