On 21 June, 17:26, Fred wrote:

A neighbour has a rusty metal gate that he would like to replace. It
is fifteen feet wide but the few places he has asked only supply
replacements up to 12 feet wide.

There's a reason for that...

How easy is it to make a wooden gate that size?

Not.

Go for two 7 1/2' gates instead - much easier all round.

Buy galv steel. Lighter, cheaper, stock item.

Otherwise build a 15' gate, but be warned that you need to be a fairly
experienced carpenter to do this well (ie to get a light gate that's
still rigid). Design isn't easy and the construction needs the ability
to cut some big joints that still fit closely enough to be rigid.
Although much carpentry is about pin jointed structures (stiff sticks
between floppy joints will still form a rigid structure if you
triangulate) a gate (usally) needs to be flat and so this becomes hard
to achieve if it's not to flex as you open and close it.

Material I'd choose would be Douglas fir. This is stiffer than most,
lighter than most (esp. oak) and you'll have a job to find my usual
outdoor timber choice of larch in good untwisted sections at this
length. I'd get it from http://www.bendreybrothers.co.uk (Bristol
area), and their website might give you pricing ideas.

Remember that gates are basically triangular, not rectangular,
especially when it's big or heavy and you're having to be careful with
the structural design. Use the traditional approach of extending the
hinge-end upwards, often with a "hockey stick" curve inwards and
hanging the main diagonal from that. This diagonal is the main
strength of the gate, _not_ the horizontal rails. The more angle you
get into it, the more it's working in tension and less in bending
(that's why the horizontals can't carry the load). I'd also tend to
duplicate it, maybe a 2"x6" on each side of the central rails - that
way you get the gate to be thicker (stiffer in bending) whilst not
needing to be as heavy as if you beef up the rails. Making the end
posts of heavier section (depending on what you can get) also makes
the rest of the fence look visually lighter in comparison. One trick
is to use an oak knee for the upright, the knee (a curve that grew
that way) being the cheapest way to get the shape. Weight this close
to the hinge is no problem for either moment or inertia, but it does
help to make for stronger joints.

I wouldn't make it solid, because of wind load. If you have to for
visual privacy, put staggered palings on each side.

The rest is just web searching, or a few of those lovely 100-year old
books that are reprinted so cheaply these days (Try "The Mid Western
Farmer's Compleat Almanac of Gate Building and Hog Husbandry" or
something like that, probably published by Dover or Ten-speed Press
and sold through Camden steam bookshop).

Go easy on the joints. Big single dovetails and their like are your
best bet. Complicated enough to still work by wedge action for decades
after the nails fell out, simple enough to cut well and to use large
robust components. Through bolts rather than screws (bigger than 2"
anyway) and a farmer's shop sells a wide range of galv ironwork,
hinges and latches.