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Smaller MOA dots allow for increased accuracy due to the smaller footprint that the dot displays on the target as larger dots will cover more and more of your target at range. However, larger dots are easier to acquire and quicker to put rounds down range. Selecting your dot comes down to personal preference and your specific needs.
Smaller dot may improve accuracy in rifles, but not in pistols where shooting distances are far lower and dot size does not impact accuracy almost at all. Bigger dot is to have faster target acquisition that majority of expert shooters including competition opt for bigger dots. If you shoot bullseye static competition then you may go for smaller, but for vast majority of practical uses you want to go bigger MOA. Some even use 16MOA in red dots with high accuracy. You have to remember that dot use is way different than iron sights. I recommend proper training. You are not supposed to focus on the dot, but on target. (I have been competing using red dots for some years and I am Master level).