My understanding is that smaller tea pots are considered superior. Why
this is I don't know. Perhaps because the greater ratio of surface area
to volume assures greater absorbtion of flavours in the clay. Who
knows.

My recomendation is to try to find your tea pot in an asian market. My
experience is that they are substantially cheaper there than in the
upscale or online vendors where they are usually to be found. It's
usually hit and miss in these places. You are also unlikely to find
more elaborately designed ones. I recently picked up a whole yixing set
for $25. Pretty standard, but it work. Just make sure to season it good
(see other postings for seasing tips!).

Best Regards,
Nico