IIRC xen combines seabios and hvmloader into a single 256k image somehow, so it might make sense to set the seabios image size to something between 128k and 256k. But better ask the xen people for details here.
A patch allowing the size to be other than a power of 2 was rejected. And I vaguely seem to recall that you actually participated in that?
seabios by default still picks the smallest possible power-of-two size. If you need something else you can simply set the new CONFIG_ROM_SIZE option to whatever you want, including non-power-of-two sizes.