I dont think there are any Lun size's put in writing.  It comes down to what ever the host can handle.  each type of OS has its limits.  As for your current lun's, if your planing to migrate the date from the IBM to the CX4, you need to make the disks on the new CX4 the same size or bigger.  I dont know what migration software you plan to use but that would be the minimum requirements.  If lets say you have a current disk of 20G but you know taht you are only using 10G and wont see much growth,  then you could always make a smaller lun but would then be required to use a host based migration solution that would allow moving data to a smaller disk.  In todays storage standards, 24G is small:)
We have a new CX4-120 and the plan is to migrate from an IBM box to this one. The current disk sizes the host see's range from 8GB to 24GB.  I am thinking if I should split the disks up to smaller chunks. Is there a size I should stay within. What does EMC recomend for LUN size for a total 12TB box?
LUN size in clariion