Pairing MA 5300 with Large Advents for 900 sq ft apartment?

This is a great question for MAC Labs and, as a MAC owner myself (actually, the MAC owns me), I would recommend you email this question to them. In my experience, they have always been very helpful with just about any questions you might have about their equipment.

That being said, with the MAC MA5300, I would strongly recommend you stick with 8 ohm nominal load speakers. A 6 ohm nominal load might be as pleasing, as long as it's relatively efficient or sensitive. However, all things being equal, I think 8 ohms is the way to go with that amp. Your amp can, of course, drive speakers with more resistance but won't get everything out of speakers like that, that they are designed to deliver.

much will depend on the building,  and how thin the walls and floors are, not simply which speakers you like best, but which will be more "apartment friendly"...I know, it's hard enough to pick a system without wondering about your neighbors...

If you will be using Roon, take the big ones. :)

@jl35 

Do you know if having area rugs and curtains will help both the acoustics but also reduce the sound transmission to the neighbors? 

Rugs and curtains absolutely help noise transmission. In fact many buildings require you to have carpet/rugs