I used to say eyeball it but I need to make beads exactly a certain size and that is only achievable with a press or roller. I much prefer a roller -especially when you have a pattern on it.
I got this one from Malcolm a while back (when it was $27).
http://www.artcoinc.com/marble_molds.php While I love the CG bead rollers, this one covers more sizes from small to large. I also needed a handle and so I attached one -easy peasy.