Welcome to the site Art.
I don't remember the bit size that I used, but it was just a little bigger than the bolts. Before you mount it, you may want to do a search on the spoiler being mounted about 2" back from where the dimples are. It looks different, and you may like it or maybe not.
I would put a filter on if you don't have one now. There was a filter that Subaru put on some of them that is just below the battery. If you have one, I would replace it if you don't know when it was replaced last. If it is not there, you can buy one like the PermaCool filter which screws on like an oil filter. Good luck.