I've been struggling with this too. If you put Quartz composer files directly in the Modul8 library they are rendered as 30sec long and 640x480, no matter what.
But I finally found the workaround, hurray!
If you have Quicktime Pro (as you should - it's sort of a vj's swiss army knife) just
1 - open your qtz file with Quicktime pro. Then
2 - resize the viewer window to your desired size. You can hit cmd+I to view the current size in the inspector.
3 - Save the file as a .mov using 'Save as' where you have the option to set another duration eg. 60 sec.
Now you can open the mov file in modul8 and it will have the size and duration you want. Note that even though the file is called .mov it is still a Quartz composition. It's just been wrapped in a Quicktime packaging.
Also note that if the size is too big you won't gain better performance even if you scale it down inside modul8. You'll have to scale down the mov file until you find a size that works.
Hope this works for you