Dimensions for a MOXON rectangle can be very easily calculated thanks to the existence of a handy software program called the "MOXON Rectangle Generator" which can be downloaded below. This program takes into consideration wire size, allowing for extremely flexible designs. MoxGen will show you the dimensions and (optionally) generate an antenna model file for a 50 ohm Moxon Rectangle antenna, given the design frequency and wire size. The program is written by Dan Maguire, AC6LA (visit Dan's excellent website for additional software) and is based on an algorithm developed by L. B. Cebik, W4RNL.
Download MoxGen.zip (includes instructions, 2 total files)
Download MoxGen.exe (single file executable)
Important: If you see an error message about "COMDLG32.OCX" being missing or not correctly registered you should download the MoxGen with OCX package (315 KB) and follow the install notes found in the ReadMe file. If you see an error message about "MSVBVM60.DLL" you will have to download the complete Visual Basic runtime library. This is a large file so I don't keep a copy on this site. You can get it directly from the Microsoft download site.
Steve Hammer, K6SGH