I'm not sure how much I can help but I think it should be a full size spare.
I also have a 1999 230xlwt and mine also had no spare when I bought it.
I think mine had been altered by some previous owner and I'll explain why I say that.
I bought mine from a dealer so didn't get to talk to the previous owner and therefore have no history on it.
I think that originally from the factory, all 6 wheels were the same offset so any wheel could be used in any location.
Someone converted mine to aluminum wheels where the fronts are not the same deep dish offset as the rears so the front wheels cannot be used on the rear.
Because of that I bought a stock Dodge van steel wheel that fits the front and put that under the rear (stock spare tire mounting space) so I have a front spare. I also bought a 12v compressor and tire patch/plug kit in case of a rear flat....
My logic was that I could limp on one flat rear if needed - not sure how well that would work but, knock on wood, haven't had to test the theory.
Anyway, if yours has the extreme offset front wheels that are the same are the rear, you will be good with one spare.