Answer
Change that field's Control type to markdownEditor for a much larger, more usable editing box: in the Entity Configurations screen, open Page Designer, expand All Pages until you find 'Impl Type Select Group,' select it, scroll to the IMPL TYPE SELECT GROUP section, and change Control to markdownEditor, then save and publish. Custom attributes created afterward get this larger editor for their Select Statement — just avoid actually using any of the Markdown formatting options it also exposes, since they're not meaningful for a SQL statement.