If you go to Help --> About, your WLM version will be in there. If you are not an official BETA tester for WLM 9.0 you should not have it and I would suggest removing it and installing WLM 8.5.
Anyway... IIRC WLM only has certain sizes of the DP and so can only display them at that size, you cannot make a custom size, but I may be wrong