Red dots are FUN.

For a handgun, I wouldn't go smaller diameter than about 30 mm.
Too much target obscuration andnecesity for precise alignment, resulting intoo much time to acquire thetarget. You want to be able to see the dot as you raise the gun to eye level without having to reorient the gun first. Somebody makes a 50 mil, or used to. C-more also makes some good ones that don't even have a tube, just a pane of glass in a frame.

I have a Tasco Propoint 5 for my 44mag deer handgun. It's decent. Not the absolute best, but I like the adjustable dot size, scope diameter, and it holds up to plenty of recoil from those 240grain SJSPs. Not bad for $100 at a gun show. The variable size dot can also be used to roughly range a target if you know what size it is(MOA) and how to use it.

Now that Illinois has changed the hunting regs so, when hunting deer, we can carry a slug gun AND a handgun during the same seasons, I'll probably return the iron sights to my 44for use asa'30 yard'gun for deer, and toss the dot overon my 10/22 for some high-speed plinking.

Dots really are a lot faster and easier to use than iron sights. Nothing to line up, and the bottom half of the target isn't obscured by the sights. The military is using quite a few dots these days...mostly Aimpoints, and ACOGs. Law enforcement also seems to lean toward the EOTECHs, but that's a heap of $$$.

Have fun.