[Douglas Brett]

It’s a little late I know but I have only just found this forum and my answer might be useful in future to someone who asks the same question. The easiest way to find the diameter od a circle (or the size of any other object) is to check the Transform palette. You can then do the sums if you like but much more accurate, and not involving any equations, is to use a script. Google aics_scripts_en and you’ll find a suite of excellent scripts by Hiroyuki Sato. Not only is he pretty good at js but he is also a generous hearted soul and these routines are free to all. Amongst the fun stuff are several massively useful routines, my favourite is Round any corner which regularly saves me hours of work, but you will also find Divide which will answer the original question here. Alternatively you could simply use Circle which generates a circle with a specified number of anchors.