DJoel, people go with 24"x36" for a lot of their "starting points" as that equates to a decent "movie poster" size.
Keep in mind that if you want to make a real improvement in your room acoustics, you will want decent coverage percentages, thus the reason why people also go with things bigger than 24"x24". Not saying that you can't do that though. I almost did (I wasn't going to go with, and didn't go with, movie posters for are). I was looking for a certain amount of wall coverage to improve my acoustics and ended up with six 24"x48" panels on the side walls (three on each wall) and an even thicker 24" x 48" panel mounted horizontally on my rear wall. At first my wife thought that they were big (so did I in fact), but we like the look now that they are done, and the acoustical improvement is great. I don't remember the exact estimated guidelines for how much wall (plus floor and ceiling) coverage I was shooting for based off of my own room acoustics, but I could have easily had 2 if not 4 more 24" x 48" panels and not be near "too dead" but have a few more reflections tamed. But, we had to draw a line somewhere without having some serious aesthetics issues.
Not saying that everyone has to do that kind of thing, but just be aware that once you put up an acoustical panel or two, you should start to notice a difference, but you can just as easily kill off too much high end and no middle or lower frequencies and not like the new sound too.
I don't mean to scare you, just help you make the best decision you can. It is your space, so you still need to make all of the factors work for you. If going for movie poster size, and knowing and experiencing what I do now, I would go for 24" x 36" with 2" thick acoustical material (Roxul Safe N Sound, OC703, etc) and the same 1.5" air gap behind that. On a 20' long wall, I would put 4 of these on each wall if not even 5. I would still go with a larger horizontally placed rear panel like I have now at 24" x 48" x 5.5" thick (using 4" acoustical insulation). Yes, an even thicker panel would be nice, but with printing limitations, you can't really go any thicker for the panel. If my ceilings were taller (mine are actually less than 8 feet, although they don't feel low in the room) I would put up two "cloud banks" or even flush mounted panels in front of each row of seats. These pieces, in conjunction with my front wall behind my speakers being fully treated including corner bass traps, would yield some great sound just slightly better than what I acoustically have now. I have no regrets though because I have my limits for placement, etc. Look at your space and see what can be done. If you end up with even just 1 panel, I certainly won't say that you are doing it wrong or anything. I will leave that for the acoustic snobs in other forum threads. Here, in this DIY thread, everyone is welcome to experiment and share what works for them.
Best of luck.