I don't have a direct answer to your question, but we use the el-cheapo aquarium pumps from Walmart and they've worked great. I would bet they were for 20 gallon tanks since they're pretty small (about the size of a beer can). Keep in mind that the limiting factor on your flow rate will likely be the tubing and air stone. So, buying the larger air pump may not result in any more flow.
Another point to consider is whether there is a filter on the inlet to the air pump. On ours it looks like a thick wad of white cotton stuffed into the suction.