speaker to big?

It depends on how much importance you place on the presence of deep bass info vs the insertion of an add-on parametric equalizer.

Personally, I would opt for a 'fuller' range floorstanding speaker and lopping the tops off low frequency nodes caused by room dimensions (or necessary set up compromises) than relying on minimizing room problems with small speakers or room treatments for a couple of reasons.
1) Some of the room/bass problems can be well above 60hz, well within the effective response of many 'monitors' and can really muddy up bass response.
2) It is very difficult to treat a room to eliminate or minimize deep bass problems related to room size. Resist buying into all of the hype hear about band aid products. DO some research into acoustics for your self.

FWIW even if you can't set up the speakers that you decide to buy for your room, try to find some of similar size and frequency response and listen for yourself to see if they create any problems (for you, not just in theory).

FWIW.

"Loud levels" can be simply overdriving the room. Consider that the initial signals from the speakers are continually bouncing off walls, ceilings, etc, without sufficient opportunity to decay. This will destroy good 'imaging'.

You can help this some by deadening the room BUT then when you play music at normal levels it might sound a bit dead and uninvolving because you may have overdamped your room.

You've got good speakers - I doubt that they are a problem but it might be informative (to you) to have a SPL meter and test record and see exactly what your frequency response is. Knowing that might lead you to a solution, or not.

But, IMHO, if you want 'loud' and imaging you need a much larger room with better dimensions and very selective set up, including acoustic treatment of problem areas.

FWIW.

FWIW, with panel speakers and electrostats, you need a fair amount of space behind them to get the best imaging available. Too close to the rear wall and you will be combining the first reflections with the primary signals which will blur sharp images and may seem bright as well. I set up Quad's about 4.5' in my 13.5x19x9 room and still had to use diffusion materiel behind them.

I've never brought Maggies home for a listen but from what I've read they like power and sound best at higher volume. They are excellent speakers if set up right in the right room but I suspect they might not be a solution for your problem. :-(