[jshilan]

Let me start by asking a few questions.

Do you need a gate?
Why do you want/need a 5' gate?
Can you do a 6' instead (two 3' gates making a six foot opening)
If you want future truck or equipment access why not just go for the 8' opening (two 4' gates, one with a drop road that stays fixed until you need it).

I'll assume that you are making a gate out of wood. There is no real limit to how wide a gate can be. They just get pretty heavy the wider you go. The two limiting factors are the hinges and the weight of the gate itself. In order to prevent the gate from sagging you need a cross brace or some sort of cable (or both) to keep the fence from sagging.

As far as the 6x6 post goes, that is a definate. You need the added strength and width for the hinges and just to keep things in scale. Both gate posts should be set in concrete since they get a lot of use. Most line posts can be set in tamped soil.

If your not sure how to build it go to a fence company and look at their details. You can also go online and look at walpole woodworkers. They build beautiful fences, stuctures etc and are more substantial then your local installers.

Before you do anything, price out a gate from a fence company and then price out the cost of the materials to do it yourself (hardware too.) You may find that unless you are really into building a gate that it might be worth it to have a fence company supply the gate, posts and hardware and then install it yourself. You'll save a few dollars and actually complete the project. Plus you can tell everyone you built it. It'll be our little secret.

Another thing to consider doing is building an arbor (or mini pergola) with (or without) a gate. These have four posts and are much more substantial, structural and are really fun to build. You don't need to do an arched top. I attached a picture of one I built years ago.

BTW - the website you listed has some good info. Depending on where you are I'd go down 3' (not 2). Also check out Better Homes and Gardens Magazine they have plans you can buy.