Thanks Lori and Marco.
I've successfully set the limit for a custom global parameter but I am struggling for an inbuilt PTC one. We use "PTC_COMMON_NAME" for our descriptions. Can this be length limited in a similar manner? If not then I can set up a "ghost" new global parameter called, for example "TITLE" and then have a relation that sets "PTC_COMMON_NAME" = "TITLE".