My penis size is about 6 inches how do I know what size condoms to buy anything on the cover or codes that say?

Trial & error. At a dollar a dozen in some stores it is not like you will break the bank to find out what fits. You should invest some time to sort this out starting with a standard size. Look for the use by code & remember they can fail if out of date. Make sure they are tight enough to prevent leaks but not uncomfortable. If you need another size try several brands until you find one you like.
Use normal size. Use the standard "size". There are no sizes. It either comes as standard or "magnum".