My wife got me a greenhouse for Christmas.
According to the specs on the Agri Supply web site, it is 55" wide, 57" deep and has a center height of 76". Assembly wasn't difficult, but it was time consuming. I think it took me the better part of 90 minutes to put it together. The entire thing is made out of metal tubes that fit together with plastic joints. All of the shelving was included.
You can see from the picture below that there is a fair amount of space inside. The white tub on the floor contains 17 gallons of water. This (in theory at least) acts as a heat sink. It gets warm during the day and releases the heat at night. I also have a heater and a small fan in there. I picked up the heater at Home Depot a few years ago. They were ~$50 or so during the winter. I picked one up on clearance at the end of the season for $12. I couldn't turn it down for that price. I never really used it until I got this greenhouse.
Here is what automates the system and allows me to keep my hands off. I have a green extension cord running into the greenhouse. Into that I plugged in a 3-way orange adapter which I found at Harbor Freight for $1.99. Walmart, Lowes, Home Depot have them too, but Harbor Freight was the cheapest. Moving on, I installed two thermostatically controlled outlets. The grey one comes on at 35 and goes off at 45. The heater is plugged into it. The fan is plugged into the red one which comes on at 78 and goes off at 70.
Here's a better view of the shelves hold various containers.
This temperature and humidity sensor allows me to monitor what is going on from the base unit in the house. I don't remember what the ambient temperature was outside that day, but you can see that it was 95* in the greenhouse. I've seen it get as high as 112*.
Here's the bin of tomato seeds that I started in the garage the other day. They seem to be doing OK so far.
I haven't been using the greenhouse for very long, so it'll be interesting to see how well it works.