when you aren't doing 18 x 24 posters what sizes prints are you doing and what screen sizes are you using?
I'm wanting to move up a size ( I'm using 16 x 20 the largest i've printed has been between 8 and 11 inches )- I'm not ready for 18 x 24 at this point - and am looking to see what might be a good idea for art prints.
I've used 23x31" to print on 12x18" paper for a long while. It works really well for that size. I don't think I'd want to go to a bigger paper with that frame size, though. It seems 23x31 is the largest frame typically stocked at most suppliers.
I use 20x24'' frames, as that's the max width that will fit with my current setup (a manual 6/4 shirt press, with one arm & station converted to use a vacuum platen). The largest print size I've done is 15x19'' (paper size came out to about 17x21 after deckling edges), but it was just a 1-color design and still a huuuge pain to get good tension/contact, and ink was basically spread to all edges of the frame. Most prints I do are on 13x19'' paper, so around 11x17'' actual print size.
I print with 23x31 screens almost all of the time. If I have a much smaller print that I am working on, anything in the 9x12 neighborhood, I just put 2 images/color on the screen and tape off the one that I am not printing. I also print 13x19 image size prints routinely with that size screen, works perfectly.
I agree that I would not go much larger image size wise, at least not much wider. You might be able to get away with 14" wide, but not much more at all.
I used to print 16x22 paper size with a .75 in border all the way around...most of the time is was fine...but its probably not recommended. I just started using 25x36 frames to do 18x24 paper with 1.25 inch border all the way around.