As far as I'm aware, you can't control the height and width of an image in a post using "Forum Code". If the message board allows html, then of course you can use the standard image tag.
someone posted a mod for setting up image size in the forum code. This was like awhile ago. I remember testing it out and it worked. Look for it in Mods w/ code. If you can't find it there, it might have been archived.