I'm still yet to install mine but what I've bought is a set of rivnuts, small washers and socket head bolts.
The length of the bolt shouldn't matter too much as long as it's long enough. A little too long should be fine as they bolt up quite high so tyre contact won't be an issue. Just keep in mind that if you do put in shorter bolts, you will need longer ones for the bottom corners of the flare; the two that go under the indented line on the body of the car and the bumpers.
This has been said before but just to go over it again, rivnuts are threaded rivets which you can punch into your fender. These will make taking the flares on or off (for whatever reason) SOOO much easier as it acts as a captive nut. Just make sure you don't do the bolts up too tight as fender flares are just fibreglass and aren't very thick.
If you are a noob like me though, your best bet is to go into your local nut and bolt shop and get them to come out to the car with you so they can tell you what bolt length to use. This helped me a lot since I'm not very up with bolt length and thread sizes.
One thing to keep in mind is that you cannot load rivnuts into a basic rivet gun. you'll need a 'rivnut gun' which is quite a bit more expensive. Put a word on the forum and see if there is anyone local who can lend you one, otherwise just hire them.