Here's a link to one I made a while back, it's over on IFI. http://www.iforgeiron.com/topic/40861-new-gas-forge/#comment-416980 scroll up to the top of the page.
Size wise, just work out the size required to accommodate the largest thing you'll be working on, add four inches for the Kaowool, or whatever insulation you're using, and build your burner, or burners to suit! If you're in the UK, the best burners bar none are built using a part made by Amal of motorcycle fame. To save waffling on here about them, if you're in the UK, pm me for more details.