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As people who have had the "container graphics tear" graphical bug know, the container grid system runs on a 5 x 25 grid, so containers themselves can be resized along those lines (a lot more vertical than horizontal), but I have yet to see any indication of being able to resize effectively larger horizontally.
Default seems to be 5 x 4.5 grids, SA Copper and I believe Legacy II has 5 x 5 grid and SA Bitter Black has according to the screenshots I've seen 5 x 5.5 grids, so some amount of resizing is possible, but only within the 5 x 25 matrix it seems.
I'd gladly be proven wrong though!