as the heading says....
how can you determine the size of the engine from the outside without stripping it down to measure barrels etc.
is it to do with the engine number?

the letter/s before the number 2 my knowledge, thats if its a factory spec egine still anways

I have the "idiots guide" which tells us that stuff. You do need the chassis or engine number from rego cert. Post it and I can tell you.

With type 1 engines, the engine code only really tells you what the case is. Most VW engines would have been rebuilt by now, so you will commonly find 1300 or 1500 engines with 1600 barrels and pistons because the fit with next to no modification. The 1600 barrels wont fit 1200 engines though.