I've had to do this before using the textbox with no lines/no fill
technique that Suzanne described. The textframe within an autoshape is
the largest rectangle that will fit within it's margins. So
non-rectanlinear shapes such as circles can really limit the amount of
text that will fit in them.

The one tip I can give you is to enable the drawing toolbar. On it,
select Draw - Grid and adjust the grid to tighter coordinates. This
makes centering the textbox within the circle easier. When you get
everything looking the way you want it, make sure to select the two
shapes and group them to prevent shape shifting.

HTH,

Nick Hebb
BreezeTree Software
http://www.breezetree.com