reference geometry is infinate, in so far as when you place a plane in vertically above the top plane, the planes extend infinately in both directions. same deal with axis. infinately long. the size of the plane in the viewing window is goverened by the thing it is referencing. so you only need to create the plane and then drag the points arounds its boundary to the size you want.
to create a plane at an angle and offset, you need to create a plane on an axis or sketched line at an angle to another plane or face and then offset your other plane from the first plane. (hope that makes sense).
so if you wanted a plane at 45 deg to the top plane, but offset 100mm from the origin, create an axis at the intersection of the top and front plane (or right plane depending which way you want to offset it) and then create a plane using that axis and the top plane. make the second constraint an angle constraint at 45 deg. then offset another plane off the plane you have just created to 100mm.
hope all that makes sense.