What general size are you looking for? No, that's not a joke.
Looking for a "hanging off the side of the house" pole? Usually about 3 feet long. I'd prefer the solid wood versus the 3-section thin-wall skinny steel pole. Why? Because the skinny thin-walled steel pole fails in a moderately strong wind with a flag attached.
Looking for a vertical "front yard" flag pole? Unless you want to spend big bucks for a one-piece pole, you're almost stuck with a sectional.
Oh, and budget usually matters too. If budget doesn't matter, get a one-piece pole made of stainless steel, titanium, or maybe HA aluminum. Yeah, that's it, a 20 foot tall HA aluminum flagpole. With a 24k gold-plated eagle on top of the ball at the top. And a nice solid footing for it, let's say 2ft by 2ft by 4ft deep for typical soil conditions, deeper for poor ground conditions. Order 2 yd3 of 5000 psi concrete. :nana: