Most any flint sold as rifle size should work, (generally "musket", "rifle" and "pistol" sizes) but you want it to be about the same width of your frizzen. Smaller will work, larger will tend to smack the barrel or will look funny poking out the starboard side of the cock-jaws. But, it is not critical, and during the learning curve will be less critical.
Black English, and French amber are my favorites. I'd avoid the "machined" or cut flints, that are not actually flint. Track of the Wolf has a large variety and they ship pronto. ! Any bunch you buy in any one size will vary in size. I like to get as wide a flint as possible (within reason) in the jaws of my flinters.
Anyhow, order a bunch of them so you can really pick and choose among them for the ones that fit and spark best in your lock. Again, that will be a long learning process.
Dang...you got a long road ahead of you.