Complete Red / green dot Noob

[eighteenninetytwo]

Looking for a red / greendot sight for a 9mm carbine due to worsening eyesight. I have never used or bought one of these before, so I'm probably going to buy one from someone like Sig (found a Romeo sight on amazon) who I'd expect to be dialed in on sights for pistol calibers. there is such a ridiculous amount of info on the web about them, so I thought I'd ask here if there is any reason that this won't serve for shooting between 7 and 50 yards? Also is the 2 or 3 or 5 MOA the SIZE of the dot itself, and should I bother with magnification for these distance?