I'm looking at the un-released Workbench 3, but I think it is the same for version 2.

Open the Preferences panel, Under General/Appearance (open the disclosure triangle) there is an item Colors and Fonts. I see an item Text Font, which is currently Monaco 11 on my system. You can edit that to whatever you want -- with the warning that you should not use bold or italic.

If you change your mind, there is a Restore Defaults button.

There does not appear to be a way to alter the Eclipse UI font size.

There is a discussion in the Wolfram Workbench User Guide, which is accessible under the Help menu. On my system, this opens a document in your browser.