I have a VM used as a file server. Because of the size of my datasets, it has 3 VMDKs created as a spanned volume inside Windows. The VMDKs have been created as 1.99TB. It's been working fine. I just installed VEEAM B&R 6 and it refuses to back up those VMDKs because they're too large. I found that they should be 1.9TB.
How can I resize them without affecting the data inside Windows? I don't have the space to create a new 6TB disk.
I believe the limit you're seeing with Veeam isn't with Veeam but with Windows. If you convert your backup location's disk to GPT instead of MBR, you should be able to backup >2TB. The MBR limit is ~2TB.
Did Veeam Support tell you 1.9TB was the max? The only thing I've been able to find on their site in relation to that is this, which relates to VMFS3, I believe: http://www.veeam.com/KB1091
Unfortunately there's no way to shrink a virtual disk other than cloning it or migrating/copying the data to another disk.
Btw. the maximum virtual disk size - in order to be able to create snapshots - is 2,032GB (see "Calculating the overhead required by snapshot files" at http://kb.vmware.com/kb/1012384)