It's not thimble-sized.

"The current show pool is 36 feet deep, 180 feet long and 90 feet wide, and holds 2,491,400 gallons of water. The two side pools are 15 feet deep and 150 feet long and 80 feet wide, and holds 1,058,500 gallons of water. The closeup tank, where the underwater viewing area is located, is 30 feet deep, and the medical pool is 8 feet deep, and holds 67,600 gallons of water."

From: http://orcafreak.webs.com/facilities.htm

I don't know how comfortable that is for the orcas, effectively, but it's not "thimble-sized." A thimble holds less than one ounce of water. I find it impossible to form an opinion on this issue because of the half-informed emotional hyperbole from both sides.