I'm thinking of getting a belly putter to help as a training aid and maybe a short term putting improver on the short putts.

What length would it need to be (I am 6ft tall), or is there an easy way of measuring this ?

I think they are in the region of 41 or 42 inches as standard, but obviously you'll need to try to see what suits.
Not sure if it would do your game any good though. What do you expect to learn from using it?

Just finding out ideas. Actually, my first "port of call" is to get a putting lesson and practice some drills for a while.

I just thought it might be an idea to try one out to stabilise my putting stroke and give me some confidence on the 6ft putts that I would like to consistently hole in 1 putt or so...

Dr Gonzo, I was having a nightmare on the greens that was starting affect the rest of my game. Went to a Ping Doc 15 belly putter last summer (Oh what fun my playing partners had, "Looks like half a dustbin lid" etc, etc.) What it did was give me back was some confidence, especially from 6-8ft range. Went back to my lovely Scotty Cameron a few weeks ago and so far so good. I would recommend you have a go with one and see if it works, it did for me.

Most of the Pro's who went to the belly putter VJ (I wish he would go back to the belly putter as he is hopeless with the short putter) and Monty have gone back to a short putter after using the belly putter to get their confidence back.