The conduit catalogs in AutoCAD MEP include sizes up to 6" for Aluminum and Galvanized conduit, however, the default part routing preferences only allow routing of sizes up to 4". This is because the transition fitting in the preference is only avaialble up to 4", and therefore, restricts the available sizes.
If you remove the transition from the preference (or modify the transition to include sizes up to 6") you can then route 6" conduit.
The embeded video demonstrates how to modify the preference to remove the transition. Note that it is likely you will need to save/close/reopen the file after modifying the preference.