The simplest way to do this is to work with pixels as your units. I assume that the majority of your work is produced for display either on your monitor directly or within the forums, so pixels would be the most natural medium to work with.

Create your image, then when it comes to exporting it to the forum, simply resize it to 600 pixels ON THE STAGE. (You might have to resize the other dimension to a whole pixel value by turning the "Lock Aspect Ratio" off, and typing in a whole number value without any decimals.)

Then export. This has the advantage that what you see on your screen is the same size as the exported image (Viewed at 100%)

The reason for requesting that images are kept to around 600 pixels is so members using lower resolution monitors don't have to scroll, especially horizontal scrolling.