IMO, 5 gal is waaay too small.
Most mashes I do are 4.5 gallons of water. That would leave you 1/2 a gallon of space for the grain, which is just not gonna happen.
I have a 40 quart cooler and I've done up to 25 pounds worth of grain in it. With the beers you want to brew, a 5 gallon mash...