Q:

Recover files from a failed RAID 1 - How do I rsync if source file size is bigger

I have 2 drives that were on a mirrored RAID. The server crashed (failed array) and there are no backups. The client was setup with Carbonite, but failed to renew their plan, which expired this past July. Now they're stuck with a down server and no backups.
I pulled the drives, both of which appear when I connect them individually to my computer using a SATA to USB adapter.
Both drives obviously have issues, but I've been able to recover most (almost all) of the files off of one of the drives.
The recovery destination is a NAS in my office. 
Now, I would like to attempt a recovery from the second drive.
I think it makes sense to ONLY overwrite the files on the destination NAS if the source (2nd hard drive) file size is bigger. My theory is that if files were corrupted on the first drive but not corrupted on the 2nd drive, they would be smaller (and vice-versa for good files on the 1st drive that are bad on the 2nd).
Is this a stupid theory? Should I just stop acting like an idiot and create 2 separate directories for "disk 1" and "disk 2" and let the client sort it out?
If my idea is sound, how would I go about this using rsync? I know how to use rsync in general, and I know how to use rsync if the file sizes are different or if the file timestamps are different, but I don't know how to do a conditional copy if (and only if) the source is bigger than the destination.

A:

I think it makes sense to ONLY overwrite the files on the destination NAS if the source (2nd hard drive) file size is bigger.

File size would typically be the same; the filesystem tracks file size and corruption won't care about file length boundaries.

Should I just stop acting like an idiot and create 2 separate directories for "disk 1" and "disk 2" and let the client sort it out?

Best to leave data recovery to the professionals if you're not quite sure what you're doing. No article you read online or answer you'll get here will give you the background and experience needed to do a full and proper recovery. If the data is critical for your customer they need to pay up and get it professionally taken care of.

