Speaker size and soundstage

You have gotten very good obervations;

Flat repsonse speakers, on and off axis
speaker repositioning. The record mixer was likely mixingat a nearfield, classical 30/30 deg traingle position
room character upsetting that flat speaker
head position aligned with the tweeter and/or mid intersection
first wall reflections along that same ear level blur the original image
near field = more direct orignally recorded clues and information versus more reflections from your room

I have had 7' 6" line arrays and stand mounted two ways. The points above make the image, not the speaker size. Who listens 6" from your ceiling?

And we aren't in control of the recording Mixer's sound field positioning. Sometimes they try to make that flute stretch 8 feet between your speakers??? 

Happy hunting.

Magnepan 1.7's would do very well in your room. Soundstage,  Dynamics, all there. 

Check out Tannoy's. After a year of moving things around and learning more about speaker setup, I finally found the magical position for the Turnberry's. Since then, it's been heaven. Nice wide and tall soundstage and I have depth. Imaging is excellent. 15x20 room with not a whole lot of room treatment. My listening area is open and no clutter near the speakers or in front of the listening position. Gear is also down low and horizontal. It makes a difference. More room treatment and solid doors are next on the to-do list.

I agree with those who have pointed to adding a pair of Subs to increase your Soundstage, for your situation. Getting another brand of speaker in the same price point is more of a lateral move than an investment in achieving your goals.

I have speakers that perform very well within a frequency response between 30/28Hz - 40KHz. That said, after a few years I found myself wanting a more dynamic soundstage without breaking the bank. By adding a pair of REL subwoofers, I was able to extract deeper, cleaner bass,  fleshed out detail in the midrange and greater sense of spatial separation. I would guess that the low frequency is about 20/22Hz.

Good Luck

Lyndonbell. You are asking about “Crossover settings” on your AVR. YouTube had excellent vids on this subject.   This forum is full of nitwits who won’t give a straight answer.    

As to speaker size, large floor standers xylene to have a rich full sound especially for large rooms.  That is my wife’s preference.   But nothing to do with soundstage and imaging.  Personally I think Sonus faber make some of the best.  https://www.sonusfaber.com/en/products/olympica-nova-i/