Hi Southerncomfort I like approx 6X6 and about 6` tall so you can stand and move around a bit as your little one gets bigger it will work . You may find he will fall asleep so a little room is good my 2 cents. Good luck and hope he will learn to hunt.
I built two from heavy 6in metal studs 18ga. for the legs braced them with 20ga. metal framed the floor with 20ga. decked it with 5/8 pressure treated plywood carpeted it with outdoor marine carpet framed my walls with light 25ga metal same for the roof put 3/8 plywood on the walls an corugated metal on the roof. I made one of them 9ft to the deck the other 12ft I built it laying on the ground an then stood them up in place I built them in two pieces them in two pieces the base an the box put them togather after transporting them, in all I probably got 100-150 dollars in them but I got most of the material from my job so actual cost to me 0 dollars.