What are the technical and practical size limitations of raw disk files to be used with KVM? Can I create a 2 or 3TB raw disk file as a data drive for a KVM based windows virtual machine (acutally using Proxmox as the hypervisor) without problems?
I'm not aware of any restrictions in KVM regarding the disk size. If you use a raw file as a disk, you have of course the limitations of your filesystem (ext4 is usually limited to 16TiB per file).
I would recommend however to use a LVM partition for your virtual machine. It has less overhead (better performance because no filesystem layer) and you can extend it easily.