Set boot resolution in /Extra/com.apple.Boot.plist. You should probably also change the resolution of the theme you're using, you can edit the plist inside the theme folder.
If it doesn't work, your video card BIOS does not provide a 1920x1080 VESA mode.
You can see a list of the VESA modes that your video card supports from the Chameleon GUI. Select a drive or partition and press the down arrow.