15 inches, if memory serves. I’ve had it for a couple of years now.
The external monitor is a good alternative when you are at a ‘workstation’. Limitations on HD space is also overcome by using external disk drives — giving you unlimited storage.
The question you need to answer is how much is increased portability worth? MBAir won’t have as fast of CPU processor - won’t have as much memory or HD space. Even your ethernet connection is not a standard jack! But, you get the full utility of the machine - essentially wherever you want to be (and with the new battery times - as long as you want to be there).
The ‘replaced’ machine has a few years on it - so whatever you decide will run circles around the old technology. There are software options that will allow you to ‘do’ the same things as your old machine did - in a more integrated fashion. And, as far as your old files, you can remove the HD from your old machine and have that as an external drive for your new machine (keeping all of your old files handy).