Is there any rule of thumb on size of crawl spaces? I am building a
three season 10'x24' porch on piers off the family room. The
family room has a cement foundation with a concrete floor. How much
room do I need between the floor joists on the porch and the dirt below
them? Do I need a vapor barrier? Does the crawl space need and what
size vents?

Around here (Pacific Northwest), code requires 18" between the ground and
the floor joists, though that can be reduced to 12" under beams. However,
if you ever have to crawl under the porch, you'll wish you had more space.

We designed 24" stemwalls on 6" thick footings for a total 30" high
crawlspace. Our footings actually ended up being thicker in places, so we
have a little more than 30" in many areas. I find 30" to be quite
comfortable to move around on my hands and knees, as well as dragging
pipes, insulation, or whatever through the crawlspace.

As for venting, we have to have 1 square foot of vent for every 150 square
feet of floor area. There are supposed to be vents close to the corners,
and allowance made for cross ventilation.

Of course, if your porch is built on open piers, venting won't be much of
an issue.

Anthony