I'm happy with my little 12" x 18" plate. It's large enough for pretty much everything I do and doesn't take up much room in my shop (clear flat surfaces are at a premium!). Any plate is better than no plate, but I'd be frustrated if the only one I owned was any smaller than what I have now.
I only have one thing to add to everything that's already been said: no matter what size your plate, you should rest it on three points. To go the extra mile, you can put the rests on
Bessel points, but that's not nearly as important as ensuring the bottom is resting on a flat plane. Three points guarantees a flat plane.
Even a 4" thick piece of granite can eventually get out of flat if you just rest it directly on, say, a child's desk with a thin 3/4" sway-backed wooden top. But only a complete idiot would do that.
(I finally realized the error of my ways.)
For a small plate, up to say 18" x 24", any three small pieces of metal or wood placed somewhat evenly inbound from the edges will suffice. I've just got three chunks of scrap wood under mine, but have considered using some 1" steel balls with a flat milled on one side. If I ever do go that route, I'll probably put the flat sides against the bottom of the granite and the round bottoms against the table to avoid putting too much point stress on the granite.
For a larger plate you should locate the 4 Bessel points to support the plate. Two of the points should be supported directly. The other two should be supported by a rocker arm of some sort, and then the center of that rocker arm supported. That gives you three points of rigid support to define a plane.
I understand that better plates have the Bessel points already marked on the underside, but you can calculate them yourself pretty easily. Just measure each dimension, length an width. Multiply those dimensions by 0.5594 and that should be the distance between the support points (just measure half those distances from the center lines to locate the points).
A real support stand should also protect the corners of your plate to prevent anyone from knocking it off the proper support points.
Dale Derry has a nice series on the stand he built for his plate.