The error means that the size of the image on the RME server is different than the size of the image now on the device's flash. This is bad. It will most likely mean the device will not come back up if rebooted.
This could point to a CISCO-FLASH-MIB bug, bad flash, a network problem which corrupted the transfer, or a bug with SWIM. If multiple devices are giving you this error, then bad flash is probably not likely.
When we see these errors, the problem is almost always a device bug (i.e. bug with the CISCO-FLASH-MIB).
The best way to rule out RME is to try to transfer the image manually from the RME server to the device. If the problem still occurs, try a different transfer protocol (e.g. SCP as opposed to TFTP).
If the manual upgrade succeeds, this leaves a problem with the FLASH-MIB or RME. Since, as I said, most reports of this point to a device problem, you might have to bite the bullet, and complete the upgrade manually on these devices.