Thank you MountainWoman for your excitement ... to be honest, I didn't know if I was overstepping by making my suggestion! It just seemed to me that you had everything, and were missing that one "big" item. Big, in that I don't know what I would do without it!
There have been many different threads on here discussing the merits of one type over the other, and lots of how to's, different styles, etc.
Here's a bit about my experiences .....
My initial trial was fabric held on the wall with painter's tape. I soon realized it worked, and I wanted MORE ... but I definitely wanted something more stable that I didn't have to fear would have come down as I worked with it. I tried flannelette, fleece and batting and made my decision towards fleece, because, I found that pieced squares held better than they did to the others. Plus fleece gave me a wider width to work with. I also chose black fleece, because as I went around to LQS', I saw too many design walls of batting or flannelette that were just outright grubby looking. I didn't like that. Plus a designer had showed me one time with a design sample board how black gave a truer colour. One detriment ... if you do a lot of white/light quilt work, you may not like the black. A lint roller does a quick job of cleaning it off, if/when threads start clinging.
I jumped right in, and my first design wall was a biggie 7-1/2ft x 10 ft. I wrapped fleece onto 4x8 boards of tentest, which were then mounted permanently to the wall. Tentest was chosen because it was cheap, plus would allow me to stick in pins when I wanted. I realize that you may not be able to go permanent, being in an apartment, but in my situation, I knew I was here for the long term and too, I had the wall available! I have found that pins are not needed for this wall, but I wanted that option.
I like a hard surface behind the design wall, so that I can smooth a block or the quilt onto the wall. That is what helps to keep it there for the long term! Pinnability isn't really needed, except when I want to include paper ... eg. patterns, notes, etc. I have had large quilt tops on the wall without pins for a length of time. Although I now put in just 3 or 4 as a security, just in case! All of my FWS and PC quilt blocks were displayed as I assembled the blocks and they never jumped ship! However, you may find that you want to be able to pin things ... thinking that if it is in an area where you may brush up against it, you may want to secure what you have and not be playing 52 pick up!
There are some other less permanent options, and that is what my other design walls are .... from 15" square (portable), right up to that big wall. Of course, the basic where I started and where you may want to start .... painters tape and some fleece for that trial run. If you don't have any fleece, try a piece of batting that you have instead of buying until you know for sure what you want.
Others ... I have some small boards covered with fleece that I use when piecing blocks .... they can sit right on the edge of the sewing machine as I work. All the pieces are together, and I can take them to the ironing board for pressing and return to continue on. Can be as simple as a piece of cardboard, corrugated plastic or plywood covered. One is thin plywood, with a couple layers of batting and then the fleece. Corrugated plastic is fairly durable and leaves you with that pinnability! Sometimes I use more pins when doing block work than on a full wall .... pins labelled A B C etc to identify cut parts are easier than having to label each cut piece. Or if there are multiples of one size, I can remove the two I need at the moment, and keep the rest stabbed to the board until needed later! Plus, they keep those pesky little pieces from disappearing until sewn into place. )I just did some blocks at Christmas with one inch squares .....
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The same corrugated plastic ... I covered 4x8ft sheets with fleece. They hang from the wall right up against the ceiling with picture hanging hooks. Knowing that my DJ project was going to be for the long haul, I did not want to tie up my main wall, and thus, why this wall was created. I went for the width between a window and a door, about 7 ft and covered it fully in this manner. Also, because my DJ was on white background, I did give in to white fleece for this one.
These corrugated/fleece type may be a good option for you as less permanent and picture hangers are normally allowed in an apt!
Likewise, I have some other corrugated/fleece smaller design walls that just lean up against a wall, or a piece of furniture. The corrugated/fleece is nice in that they are lightweight, easily movable and if not in use can be slid in behind a piece of furniture until needed. I was fortunate to have some "free" sources for the corrugated plastic. Keep your eyes open for old signs being pitched! That's how I have continued to add to my collection. Can you tell that I tend to work on more than one project at a time?
Yes, I love my design walls and wonder how I ever quilted without one.
And I think you will soon agree!
Your question as to size .... as you have figured out by now .... any and every size.
You will never have enough! Just get started!
I hope these ideas help and not confuse you!
Please be sure to ask questions, if I have left you totally lost!