Right click on your pic, chose properties and that will give you a size. Any Paint program or Photoshop will tell you as well. Also, go into the Babe thread and right click on a pic, it will tell you the pixel size and will give you a good idea of what is what.

Your pic should be visible side to side in the thread without having to scroll to the right to see all of it. Not all monitors are the same size or set to the same rez, but 800x600 is a good rule of thumb. (Soly's rule of thumb if memory serves.)