Likely an 8-32 or a 10-24. I would take it to a hardware store like True Value and match it up with a known nut in one of the pull out bins. Or, a good tap and die set will have a thread gage included, which will give you the thread pitch. Then measuring the diameter will give you the size. Of course, it could be metric, and the process would be the same. Or, maybe Slimline could tell you what the thread size is?