You can make the key any size you want that is a valid Access data type/size (except memo or blob, I believe).
The PK doesn't necessarily need to be a single field either.
In your example, it looks like you want an alpha-numeric key that is (always?) 6 characters. You'd make a text type field, length of 6. Click the key icon and you're set.
How you assign the values is a different story.
--Jim