Any thoughts on boat length and engine(s) related to gas usage?

My wife and I are looking to buy a boat. We're both teachers and have the summers off (not to mention a full week in fall, 2 weeks in winter, and another week in the spring). We've been looking for a cruiser in the 27' to 30' range. The bigger the boat the better, but we're also wary that a larger boat may be much more expensive to operate. We're also leaning towards twin engines, though we expect that they will use more gas. We've read that it's not quite double the gas, but we do expect expenses (oil changes, etc.) to be more with twins. Does anyone have any experience with moving up in size who could give us some basic ideas on what we could expect gas usage to be at different lengths and engines? Most of our use will be cruising to a cove and anchoring, though we would like to take some week-long trips down the TN River every now and then. Will we save a lot of $ with a 27' compared to a 30'? Will a twin serve us better (power, backup in case 1 engine fails, etc.). Any advice would be greatly appreciated. Thanks!!!