I've built a lot of fences from scratch on my farm. What I'm going to talk about in this post is stuff that I've learned. A properly placed and sized gate can make your life much easier.
Tractor/Main access gates
At the top, pointed to by green lines, are the tractor or main access gates. My experience has been that these need to be at least twice the size of the biggest thing you'll move through them. If your tractor is 6' wide, your gate should be at least 12. It would be better if it was 16. Spend the extra 15% to buy the bigger gate.
You will never notice gate width until it's too small. You will never have a gate that is too big. Believe me. I've had to replace gate posts many times. On my farm I've standardized on 16' gates for all main access points. 8' for all man gates. 8' seemed excessive at one point for me. It doesn't seem that way any longer. Cows are big.
Also in that picture above I've shown a couple of man gates; I'll put a man gate into all areas where I would have to walk more than 200' to get around the fence. I even put man gates into the perimeter fence -- because I keep animals, and they get out sometimes, and it's nice to be able to open a gate to get them back in.
Man gates and security / access control
The other thing that a man gate does in an exterior fence is to give you an easily observed access point. If you've got someone who wants to get into your property, and there's a gate nearby, chances are excellent that they'll use the gate in perference to climbing your fence. A $50 game camera placed so that it photographs activity at the gate will give you a good idea of who or what is on your property -- and for my urban semi-urban farm, I've found that very useful. If you don't offer a man gate, they'll cross your fence in a random place. Lay out a welcome mat so that you can greet them appropriately.
The picture above is a detail drawing of the herding helper gates. Imagine that the grey line in the center is your main farm gate, and on either side you have fenced pastures. By having a gate that is at the end of the driveway, you've got an easy way to direct livestock in the event they get out. That blue line might be a cow that you're chasing around. Wouldn't it be nice to just close your main gate, open the herding helper gate, and then shoo the cow in? And if you make the herding helper gates half the width of your driveway, by opening them you could move animals across your driveway with no chance of strays. Wouldn't that be nice?
The picture above is how I started building gates. Notice that the grey line hits the fence on the right. I consider that a mistake now. Here's why. If you're opening the gate, you're on that swinging end. It's pretty hard for you to stand inside the gate and open it. You end up grabbing the gate and pulling it back towards you.
If you add a short length of fence on the swinging end of the fence, you get a much more workable gate. That little red line means that you can stand and open the gate without having to move. It seems pretty silly, but when I have not built that little red line of fencing, I've really regretted it. So much so that I've redone gates to fix it - dug up the posts and reset them.
Take a look at the first picture in this blog. See if you can spot the mistake in the main farm gate as drawn.