Looking for a 300tdi hella alternator, but looks like there are different sizes of pulleys and associated belts. How do I work out what combination I need? Assuming I can’t reference off the existing!

There are different pully's for different engines, you just need the same as you have.

I don’t have one on the car. So I need to spec from scratch. So how do I know? What’s the differences?

Is it the high output type you are looking for ? If so try Hella ARM4248G.

There are two sizes for the. 300tdi, the earlier ones are larger, later ones have a smaller pulley to aid charging at low revs.
There are also two sizes of drive belt to suit.
If the replacement alternator has the wrong size pulley, they are simple to swap. If the nut doesn't want to undo a windy gun will have it off in seconds.
The larger pulley also has a spacer behind it which you need to remove to fit the smaller pulley. It comes off easily as do the pulleys.
If you have a choice go for the smaller pulley, but make sure you use the shorter belt.