It won't resize the shot for you but it's quite simple. Click Image/Resize on the menu bar & type in any size you wish. There is a selection of standard sizes like 800 x 600 you can select with one click. Then File/Save As in JPG format.
You might be better off using a screengrab utility like MultiGrab to take the shots.
http://www.mnwright.btinternet.co.uk/Then resize the ones you wish to post with Irfanview.