The larger room is much better. It's better because it's bigger, and also because the window is on a short wall. So that will be the front of the room letting you look outside on a nice day. And the window isn't at a reflection point. It's also good that the room gets wider in the rear, though that's a smaller advantage. Then follow the advice in this short article:
Larger rooms are generally better for a few reasons: longer dimensions mean a lower fundamental mode, further walls mean later and less intense reflections & also more space to put treatment and equipment. For awhile, I believed that the inherent volume mattered but I haven't found any evidence to support that; it's mostly to do with boundary distance and all its implications.
Still there are some things I'd like to improve. Especially the dip around 56Hz as well as the 120Hz peak.
I have aquired 20 pieces of 221 rockwool, 55kg/m3, 100x60x10cm. I want to create an acoustical ceiling with some of them.
For this I am considering doubling the panels, this making them 20cm thick. Also I want to hang them with small chains so I can change the height/airgap. Though I understand that spreading absorbers give more overall absorption, I guess 20cm panels help better for lower frequencies.
I'm not sure how dead my room will sound after I placed all rockwool panels.
I am still reading and learning a lot, but please allow me to fire up some questions I have at the moment.
-Will an acoustic cloud be useful to reduce the 56Hz dip and 120Hz peak? -Is it advisable to use 20cm thick absorption for the cloud? -Is it advisable to e.g. put plastic between the cloth and the rockwool to avoid making the room too dead? In theory, I guess this will make them basstraps rather than broadband absorbers. -When trying not to make the room too dull, is it advisable to use diffusion in a room as small as this?
Treatment helps SPL but not as much as you'd think. In my experience, placement seems to help that more. Try adjusting your listening position and speakers. If you don't have a subwoofer then get one. That should help improve your low end - you can even get two subs and balance their placement for an even better response.
As a side note, you're better off showing the SPL of both speakers combined. What's good here is that one of your speakers seems to fill in the null from the other. This is the kind of think to look out for when messing with your placements.
I'd say get more treatment in the room and play around with its placement. You can do a number of different things in terms of the location of treatment, speakers, and listening position to improve your response.
I can't really tell what your decay is doing because you have the graph going to too low of a dB level. Try to show just 30dB of decay from your measurement level. This is really where treatment comes into its own.
I just did a video on why thinking about your sub range before the others is not good. Watch that, read or reread Ethan's articles, search for your questions around this forum, and then you should have a pretty clear idea of what next step to take.