It has to depend on dialect, yeah.
I've only ever used "smal" to describe objects or roads. Like, something that doesn't have a lot of space inside of it? Or it's smaller than usual
"en smal blyant" = "a narrow pencil" more so than other pencils?
"en smal stripe" = "a narrow stripe".
"en smal gang" = "a narrow hallway".
I would not say a hallway is "tynn", though the walls of it could be.