Just ran into an annoying little problem using the version 5.1.12 of MySQL Workbench: the font used in the SQL editor is really small, and there’s no way to change it. The preferences dialog shows you what the font is, but it’s a text field, and even though you can type in it, it appears to be a read-only field.
To fix it, go old school and edit the configuration file. On Linux, it’s in your home directory in .mysql/workbench/wb_options.txt. Be sure to quit MySQL Workbench before editing the config file to avoid having your work overwritten.
Once you’ve opened the file, look for the key workbench.general.Editor:Font, and increase the font size to something usable. I chose 11, but some may still find that too small.
Save the file, then start MySQL workbench. You should now be able to read what you’re writing in the editor.