On my Windows box at work, I’ve used Sizer (http://www.brianapps.net/sizer.html) to resize my window panes to an exact size when I was doing screen captures. Its a great little (and free) utility that allows you to resize any window to an exact pre-determined size.
I’m looking for a similar app for OSX to use with Snapz Pro X. If anyone has any suggestions, I’d appreciate it.