It is possible to use vhd-util scan -a -p to determine the size of a VHD in Xen server. It can be done as follows

vhd-util scan -a -p /var/run/sr-mount/96ca6e70-976e-32bd-837d-f1e7bade1262/e165978f-2ff7-4958-8642-7087be83d680.vhd