Definitely bigger. If you can afford it, go at least twice as big as you think you need.
More thought at the planning stage. Plan for every thing from the floor to the roof ridge to the exact size of any axillary vents to exactly where you will place your feeders and waterers.
Unless funds is a true constraint, make it tall enough to comfortably walk in. My wife can stand up straight in ours, but I cannot. It is more of a hassle than I thought it would be.
Mine is only about 90% complete, and I am already regretting the lack of detailed plans and "shortcuts" I made. I had a basic shape in mind, a vision of the finished project and a truck load of donated lumber.
I've probably only spent $100-150 for mine since most of the lumber was donated or left over from previous projects, so I can't complain too much.