As the field is an already created one (and presumably has been populated in some records), you are not able to change it from the front end for good reason - there's nothing behind the UI to check that there wouldn't be a SQL error preventing the change, so it's simply locked. You can of course change the height and/or position of custom columns.
It's often not possible to make them smaller due to data already existing that would be truncated. It's perfectly possible to make them larger (to a sensible point), but since it's a manual SQL job and a config change, this requires it to be done via our Professional Services team to investigate and evaluate and enact if sensible. There is likely to be a charge for doing this for you.
If you would like to proceed with this request, please speak to your Account Manager or Customer Success Manager who should be able go arrange this for you.