looking to build a green house for container figs and citrus for zone 7.  would want jan temp min about 35 to 40,over 50 feb and march, so heating will be required, and maybe misting or evaporative cooling in warmer months.  perhaps 12 x 20 ft. any of my expert friends have recommendations?

If figs and citrus are your only crops you might want kind of a hybrid system. Greenhouse when freezes are the issue and open top or roll up sides with natural ventilation during the freeze free period. It really depends on your climate and whether you want the advantages of rain protection in summer. Rain protection will reduce spoilage of figs and will improve eating quality by avoiding those watery fruit that result from too much rain.

My greenhouse produces great figs and is covered all year. I need evaporative cooling in summer, wet wall and exhaust fans. That's by far the best cooling system.

Shade cloth will be more reliable than evaporative cooling, but it is also not ideal for growth. And relying on an exhaust fan is fine until the power or controller goes out and the vents close. It is a PITA to drag a greenhouse full of plants outside while it is 120 inside.

Have you considered a solar thermal design? 

This here is one of the coolest things I have ever seen "solar geothermal". One of the overlooked gems on the forum.

I bought a Riga V greenhouse and do what you're trying to do. I grow Meyer lemon, gold nugget, chandler pummelo, and Moro blood orange. In addition to my starfruit, some late/ rare figs, and carnivorous plants. I am on the same temperature schedule as you are doing too. I use 2 electric heaters in the winter( radiator and heat dish). I also installed a HAF and exhaust fan. I keep the door open all summer and the 4 vents are automatic. The fan thermostats are set to turn on at 95. I have a thermometer that sends me reports on the Internet and the highest temp this summer was 114 and average top temp was 105. I attached a link to some pics from early this season and in my other albums are installation pics. Best money I ever spent. Lots of fun.

http://photobucket.com/albums/s385/7deuceoff

The general rule is to go 3x the size you think you need  ;)   Best of luck with it.  I'd love to do that, too.

Like bob said.....whatever size you decide on.....multiply it by at least 3. Id swear that my green house is shrinking.

Guys, pictures! I love seeing other set ups.

Jason,
Still liking the Riga? Do you have a cooling system?

Yes. I am very glad I went with the RigaV. I did lots of research on the greenhouse forum on garden web. The members were very helpful especially karinmt. She had hers since 2008 and it is going strong. Everyone that bought it was very happy with it and they really sold me. The build quality is excellent compared to others in same range. The customer service was also top notch and very helpful during the installation. Prices and options vary wildly so if you go this route you must research. I bought mine from BJ's.com since I am a member and price was very good for all the included options.

I installed an exhaust fan into the rear window as well as a HAF fan for circulation above the door. They are attached to plug/ play thermostats. I was planning on buying shade cloth if necessary but I am happy with my temperature range far so this summer but August is still coming and it has been unseasonably cool for my area. We did have some near 100 degree days and that's when the greenhouse was about 114. The plants made it through fine. I just watered them and the floor and the evaporative cooling was enough.

I had electricity and a frost free water hydrant installed as well. Obviously a must if heating and cooling.

One thing I originally had was an all rock floor. It was ok but I didn't like the shifting under my feet, releveling it when I moved plants, and the rock dust. So I replaced most of the floor with 4 inch thick wall blocks( just placed no sealers) which should help with heat retention in the winter. Under the shelves and along the sides I left rocks for drainage and for watering(evaporative cooling). I am very happy with the new floor. It is more comfortable to walk on and move things around.

If you'd like some pictures of something specific let me know.

So how much did all that cost, if you don't mind me asking?

The greenhouse was $5200 plus tax. I think the extras(fans, heaters, etc.) and installation were about $2000-2500. I had the rock foundation with 6x6 wood border, electrical, and water installed. The greenhouse was assembled by me with some help from wifey. Later I installed the wall block floor by digging out some of the red rock and hand leveling it. Part of the fun for me is improving the greenhouse and making it my getaway.