Looks like a nice kettle at a good price. I wouldn't go any smaller than 7.5 gallons. 8 is a nice size where you'll be able to do some extended boils if you want. A full size kettle like that will work great for 5 gal all grain if you choose to later. Nothing wrong with doing smaller batches, but for the most part, it's barely any more work to do a full 5 gal batch.
Most of all, WELCOME TO THE CLUB!! It's a great hobby!