How big should a boot room addition to a house be?

Our 4 level split home in NW Calgary has a back entrance that is too small when my wife, kids, and I come home (maybe 3'x4'). We wanted to add a small extension. Big enough for 4 adults to come in from the cold, take off and store outerwear, while allowing others to get by. No plumbing, and adding one light. I thought 5'x'6'. I want it to be a nice feature, not just a "shed" off the back of my house. Is 5'x6' too small? and how much would a contractor cost to do it right and economical. I don't want to dig a foundation, and there is a cement patio currently outside the back door. Is it worth the investment?