Having guinness on tap at home sounds awesome. It's so low ABV and smooth I'd probably drink too much though so it would get expensive fast.
Anyway, I'm sure you're aware, but you will need to modify your setup to dispense guinness so will need more equipment, a seperate and different gas tank, different style tap, etc. The link below outlines the process.
As far as the size, I think it comes in a standard 50 L euro keg so you should be able to find the dimensions of that pretty easy but I am pretty sure the diameter is nearly the same as a standard US 1/2 barrel. You could call the place you plan on buying the guinness and ask them to measure it, but again it's going to be pretty similar to a standard keg so if you have a keggle for instance you can use that to get a good idea of how it is going to fit.