A family of Microsoft relational database management systems designed for ease of use.
I'm having trouble extending the textbox dimensions on a callout as well.
The best I can do is right click the box and select "Unlimited Paragraph" instead of "Narrow Paragraph" or "Medium Paragraph" and the textbox will automatically start to resize. Hope this helps!
Example with "Narrow Paragraph"
Example with "Unlimited Paragraph"
When right clicking the callout: