Unit options include pixels and points; however, topics only use points. Topic frames do not support font units in pixels for bubble text. If you select a pixel unit to find or replace, all of the Apply to options that pertain to topics, such as bubble text and string input options, are disabled.
If you are replacing topic fonts only, you do not have to specify a font unit.
Since you are replacing fonts in a topic only, you can leave the Unit field blank.