The windload on a piece of tubing of a given height is proportional to
the radius, but the stiffness of a piece of tubing is proportional to
the fourth power of radius.

So, as you make the tubing bigger, the stiffness goes up much much
faster than the windload, so bigger tubing will always be stronger
unless you make the wall very, very thin compared to that of the
smaller tubing.

Now, the weight of the piece goes up too, and the price is proportional
to the weight, so you don't see a lot of antennas made of 5 inch
aluminum tubing when they don't need to be.

For a freestanding vertical, you're going to get the best results with
a taper. It's good for the tubing to get thinner as you go up, because
the big problem with a freestanding pole is that the wind is trying to
tip it over. The mount at the base has to be strong enough to take
this torque. The wind has more leverage when it's acting on the tip of
the vertical than when it's acting further down, so having the vertical
thin at the top helps reduce the load on the base.

Cecil's suggestion about the Texas Towers calculator is a good one.
Also, after you put up a vertical you'll have more intuition about
this. I wouldn't make it a 1 inch vertical all the way up :-).

I had a 40m vertical, 4 elevated radials at 15 feet and 1 inch tubing
from that point up to about 25 feet, with smaller tapered tubing and
rod above that. It worked OK, but had to be guyed in two places! It
also snapped in half in a windstorm. Bad mechanical design, but I made
do because I had the tubing. It would have been a much nicer antenna
if I'd purchased the proper materials.

73,
Dan