To do so you'll need to pass the appropriate boot parameter for both the Xen hypervisor and Linux.
As an example, let's assume you want to assign 2GB to your dom0 here's how:
# echo 'GRUB_CMDLINE_XEN="dom0_mem=2G,max:2G"' >> /etc/default/grub
The above steps were executed on Debian Wheezy running Xen 4.1.3 but should translate as well to any other distribution running Xen 4.1.2 or latter and GRUB2. Users of GRUB will need to edit grub.conf and pass down the boot parameters in an analogous manner.