How big are 2.5" tyres?

How much wider taller are they going to be than 2.2 nobby nics I've currently got fitted?

Are they gonna fit my fork a 2002 Marzocchi Z1?

Depends which 2.5 tyre but as a rule they'll be 0.3 inches bigger 🙂

Depends on the tyre and the rim it's on. A narrow rim will give a taller profile than a wide one, which might make the tyre boxier and put the side knobs, instead of the top, into contact with the fork. Depending on the tyre (and how knobbly it is), the shape will be different as well.
Don't forget to factor in the clearance – if you ride a lot in mud, then it's a big deal, not such a problem if you ride on, say, the North Downs.

If it's a 2.5 high roller, it'll be about the same size as your NNs.

Cheers Glen :o)