Smaller dots like 1 moa are fainter, meaning they have less contrast with the background until you crank up the brightness which causes "flaring" or "starburst" like effects for most people. But run fairly dim, a 1 moa dot allows for more precise, slow aimed fire which is useful for small targets at greater distances where a larger dot would totally obscure the target.
Larger dots can have sharp edges (disc shape rather than starburst) and may be faster to acquire, but the larger they are, the more of the target they will obscure. Because light is being reflected off a greater area of the glass surface, the intensity of the light does not need to be as great for the large dot to be bright and of high contrast.
Medium-sized dots (2.5 to 3.5 MOA) are a sort of middle-ground.
I have not seen test results or objective evidence that bigger or brighter dots are faster to acquire. I've heard that argument based on empirical reasoning. I don't know what to believe.
I am convinced that for my own purposes, a dot between 1 MOA and 3 MOA is best. I don't see a benefit to a 6 MOA dot unless targets are restricted to very close range, even closer than practical handgun ranges like 35 yards. But I don't know whether 1 MOA is best, or 2.5 MOA. I don't have a good way to tell.
I believe red is the most battery-efficient. It also possibly avoids a blind spot that most people have in low light. This is characteristic of most people, not just people with certain types of color blindness. It is due to the normal absence of rod cells in the central part of the fovea.
But mostly I think red prevails because the batteries last longest with it.