We camp lots with our kids, and my DH camps with the Boy Scouts. In our experience, a 2-man tent is great for one guy and his backpack, and a 3-man would be better for 2 people with gear. Also, for a tarp to put under it, you can't beat Costco. Two really great ones (ugly blue) for about $15. The quality for price is unbelievable.

Good luck!