i don't think its a direct bolt-on. you will have to get it professionally installed and the size screw depends on what the shop decides is best (you will most likely have to drill new holes into your celi for fitment).
CelicaHobby.com is an independent Toyota Celica enthusiast website. CelicaHobby.com is not sponsored by or affiliated with Toyota Motor Sales, USA, Inc. in any way. The Toyota and Celica names and logos are trademarks owned by Toyota Motor Sales, USA, Inc.