The recommended thickness for glass is between 1:6 to 1:12 about for best results doing a first mirror. After you have gotten the idea of how to do mirrormaking and can control the process without wondering if you're going down the right path, you can do thinner stuff.
Then again, if you have somebody that knows the process, you can do a much larger mirror without too much trouble.
One thing is that with the thinner glass, you may want to get the glass slumped over a form to get the bigger part of the hole you need out of the way without losing glass. Don't forget to make sure that the glass goes through it's transition temp slowly so it's decently annealed or you will be having problems with the glass warping with temp.
The longer focal lengths tend to be a lot more forgiving of the hole as it's just not so big as the short focal length mirrors. A F8 would be a nice length to start with.
Good Luck!