Actually, you can get Playblast to use the exact rez of your render globals. Turn on Resolution gate for your camera, open the camrea properites in the Attribute editor and turn the overscan from 2 (the default) to 1. Then change the Gate from horizontal to vertical aspect if you need to so the entire Gate box appears in your perspective view (sometimes if the wrong aspect is set, you won't be able to see the entire gate). Make sure in your playblast options the resolution is set to "render globals". Now you can specify exactly what rez you want your playblasts to be.