The cut-out is about 6 or 6.5 inches in diam.
People have successfully mounted 6.5" and 8" woofers under there (with some creativity and a lot of work).
The problem lies in finding a decent sounding free-air sub to attach. Use this website's search function with 'rear deck speakers' as the keywords, and you will see that there has been a lot of discussion on this subject.
Hope it helps.
Brian
1998 Mystique LS V6 ATX - Satin Green/Grey Leather
Moonroof - Ford Remote Starting System
885s for fogs - Fogs with Parks
JVC KD-SH99 MP3 - Pioneer TS-A6865s in the doors
JVC 12-Disc Changer
"Sometimes I'm good, but when I'm bad, I'm even better."
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