My 5600SXT sitting atop a 54x10 tank stands a little under 63 inches tall. Thus the mounted controller adds a little under 9 inches. If I had to unscrew the controller as it sits, that would add some height requirement. Presuming you could move it out for service, that would mean that you could use one of these:
9" x 40" 1.00 cu.ft. ( 32,000 grains ) **SUSPECT GRAINS
10" x 40" 1.00 cu .ft. ( 32,000 grains )
10" x 44" 1.25 cu. ft. ( 40,000 grains )
That is from
http://www.softenerparts.com/Resin_Replacement_s/54.htm
But wait. That is seemingly not reasonable -- that 9x40 and 10x40 would have the same capacity. They may be presuming a turbulator on the 9x40.
Also note that the grains number is nominal for comparison only, and is not suitable for actual calculations.