On Sun, 21 Jun 2009 18:50:12 +0200, David in Normandy wrote:

Generally, the heavier you make a gate the more it will cost but also
the more strain it puts on the gate post. It is common for gate posts to
sag after a few years unless they are suitably strong and have adequate
foundations. A gate 15 feet wide will produce a lot of leverage on the
gate post.

Only if the turning moment from the gate is applied all the time. In
the normal position(s) of the gate the weight should be supported at
both ends, this is good for both the gate and the hinge post.

Build the gate to meet the particular requirements.

Or have made, the biggest snag in making yourself is the size and
making the joints tight enough such that the gate doesn't move due to
loose joints. We had three gates made for here, one is a large field
gate, certainly more than the "standard" 12'. It only cost a couple
of hundred quid softwood but treated and is still as good now as was
new after several years.

See what local timber yards or saw mills can offer.

--
Cheers
Dave.