There really is no such thing as a WMV 1080p file. You can force the program to make it, but it probably wouldn't play smoothly.
What do you plan to do with this file? Where do you plan to publish it? There is likely a more efficient format to save to.
You can render video as a .wmv in 1080p in other programs such as Sony Vegas. But I stopped using Sony Vegas because of crashes. A friend of mine has an older version of Premiere and he can render to .wmv 1080p.
I am uploading to Youtube and have found that .wmv 1080p has come out better than .mpeg 1080p as far as overall quality. The .mpeg format seems to be more pixely than .wmv.
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For uploading to YouTube, you'll get the best results using Share/Computer/AVCHD with the preset for YouTube HD.
But, if you'd prefer to use a WMV for some reason, you can select one of the Share/Computer/WMV presets and then click the Advanced button to create your own custom 1080p output preset.