When using Office 2003/2007/2010 in compatibility mode you may find the the bubbles for tracking changes are too small to be read. Here’s how you fix it:
1. Open the Styles Pane by pressing Ctrl-Alt-Shift-S
2. Click on the Manage Styles button in the bottom of that pane
3. In the Manage Styles dialog > Edit tab, set the sort order to Alphabetical and select the style called Balloon Text and click the Modify button
4. Change the size to something much larger eg 12 and click OK and OK to return to the document.