what is the average size of a big trampoline that I can put in my back yard?

Most trampolines sold are about 12-14 feet. It's really up to you, but I would place more emphasis on the safety aspect of the trampoline as opposed to the size of the trampoline. Make sure you can afford a trampoline with an enclosure that is attached to the mat of the trampoline, not one that goes around the padding. Visit http://www.TrampolinePartsCenter.com and click on trampolines for a wide selection of trampolines from a variety of companies that you may not see  elsewhere. I personally recommend either the Jumpsport trampolines if you have the extra cash to spend, but if not Magic Circle make some extremely strong and durable trampolines for much less.

The most important thing to decide is what price range you are looking to spend on a trampoline. Also, you must conside the size of the user and the purpose of the trampoline.Trampolines only suppose to last 1-2 years. However, if you want a trampoline that last 10-20 years, you will have to consider the size of the user and the budget. 

Larger trampolines should have a greater weight capacity. Smaller trampolines, like the 10 feet to 12 feet are more for beginners or just recreational purposes. Last thing to consider when deciding on size would be the yard. There should be at leat 4 feet clearance on each side of the trampoline or you would need to purchase a net. The size for your back yard really depends on what you're going to be using it for. Once you determine that, then you can decide on the size needed.

The average size for a back yard trampoline is about 14 feet.  They are great fun for children of all ages. I have put together a variety of most favored 14 feet trampolines....

The SKYWALKER TRAMPOLINES 14 Feet Elite Round Trampoline is a great size for all jumpers. Your family is sure to enjoy this safe and fun trampoline for years. It features innovative safety features, making it one of the safest trampolines available. Trampoline and enclosure systems feature new interlocking T-bracket technology, adding strength at each frame joint. The T-bracket prevents the frame from twisting and makes assembly a snap! The stay-put safety enclosure net (pre-installed at the factory) attaches at each spring eliminating gaps between the jumping mat and net, which are typical concerns with other trampolines. The suspended enclosure netting system protects the jumper from falling off the trampoline.

Trampolines come in a variety of sizes. The big round ones like you are talking about for your back yard comes in 12, 14, 15 and 16 foot usually.

Some have padded rails and some also come with protective netting to keep one from getting bounced off and onto the ground. Those would be the best bet if you have kids.

My children had a trampoline at their dads house and my daughter was plopped off and ended up with a broken arm. Safety is a number one consideration.

According to what size of backyard you have depends on what size you need. Here are 3 great sizes to choice from. They all are really good quality trampolines. I bought one for my kids last year for Christmas and they love it. I do recommend that whatever you purchase to either make sure it comes with a guard or can buy a guard to go around it. That will keep your kids from falling off and breaking their arm or leg.