It should be fine to delete. As a test, chuck it in the Trash, restart and play around with the machine for a while. If nothing bad happens, then you know it's pretty safe to delete. It's a bug in OPSX that has let it grow this large. 🙂
FWIW, it's a _last.log... meaning that it's supposed to be swapped out. Meaning that it's just a back file, basically, in case you should want to look at it. By the time it becomes a _last.log (as opposed to /private/var/log/windowserver.log), it is already not in use by the OS.