you don't have any raid groups that are build out of 450G drives, all RGs are 300G drives ? If the 450G drive is a member of a raid group with 300G drives, then only 300G of space being used on the 450G drive ..so if it were to fail, 300G hot spares would spare for it just fine.
One of our 300GB disk (CX4-120 System) breaks down and DELL replaces it with a 450GB FC Disk.  We get a warning message "The storage system has no hot spares that are as large as its largest disk" from Navisphere.  The hot spare disk is currently of size 300GB.  We just wonder whether we need to replace the Hot Spare Disk with a 450GB HD (Just in case the newly replaced 450GB FC Disk fails in the future).  Your advice is sought.  Thanks 
Should we change the Hot Spare Disk ?