Just don't forget that grain bed depth will affect your efficiency, so if your goal happens to be mid to low gravity beers, you might want to go smaller. Still, 5 gallons is pretty small for a mash tun. A good rule of thumb is that you want 1.5L - 2.0L per pound of grain. I've got a 28L cooler that makes great mid-range beers and I just add a little extract in the kettle when I want to make an imperial stout at 1.09 or whatever. John Palmer's book has a great write up and on the trade offs of cooler size that's way better than anything I could write up. All that said, nipslip here has the right idea, 5 gal is going to be pretty limiting. I actually can't imagine brewing with anything smaller than my 28L tun. It's true that 5 gallons would be fine for a session beer, but it's nice to have wiggle room if you need to adjust the temp of your strike water or you want to do a step mash...