My wife picked up the larger size at Fantes this past June. She prefers it over the marble when working on bread doughs. BoardSmith studied it in-depth a couple weeks ago -- give him a call and see if he can do something for you.
As far as marble slabs go, that 18x24 size is probably the smallest you should think about going. But if it will not stay out on the counter all of the time it will be a PITA to lug/ stash if you get it thick enough (1-1/2 inches?). Not inexpensive by any means, but a one-time purchase that will be a nice thing to have for many years. Call around to some counter/ tile installers -- you can probably get a piece from them for less then a fabricator will sell it for.