When comparing the sound from a set of speakers in different size rooms it probably doesn't make sense to try to put the speakers at the same distance from the rear or side walls in both rooms because the ideal distances are a function of the room dimensions; so if the rooms are different sizes the ideal distances will be different.
I've been listening to hifi for a long time and I always thought I had a decent sense of where to put speakers in a room, but recently I tripped across the formula on the www.cardas.com site (you have to look around on the site to find it). I tried the exact distances (ratios) recommended and it was pretty impressive how the image and definition snapped into focus. The formula points out that there are some optimum room dimensions (ratios)- so some rooms are going to work better than others, and there is an optimum place to put speakers in a room. Go to the Cardas site, get your tape measure out, and get your spreadsheet or calculator running and input the ratios and then move your speakers to the recommended locations and you will be very pleasantly surprised. If your room has some irregularities you might have to use the ratios as a starting point and then test variations, but even so, the ratios will be helpful. Separately, I heard from one audiophile with very capable planar speakers that he spent a long time trying to get his locations dialed in only to find that he had been moving his speakers back and forth across the sweet spots and missing by just a an inch or two - so it can be a bit tricky. In any event, the formula recommended on the Cardas site is the least expensive (free) tweak you can add to your system that can really improve the sound. - Maybe give it a try and then report your findings on this Agon forum.