You have probobly set your cubemap resolution other than default.
put it to default _OR_ increase your resolution in-game before you do the buildcubemaps
if you got the highest cubemap-resolution, i think you need to set the in-game resolution to something like 1600*1200 and then build
Your screenresolution needs to be 4x the cubemap resolution. So if you have 256x256 cubemaps (the highest and generally not necessary) your screenresolution has to have at the least 1024x1024 pixels, so that would make your screenresolution 1280x1024.