Inline fast excerpt and include macros are always 300px wide

Inline fast excerpt and include macros are always 300px wide. This is an issue when only using, say, two words.

Steps to reproduce:

1. Create an inline fast excerpt

2. Add "two words" in the inline fast excerpt box

3. Create an inline fast include referencing the inline fast excerpt name used in step one

4. Add some text to either side of the include (this is so you're able to see the extra white space)

This happens in both the v1 (legacy) and v2 (fabric) editors. Screenshot is of v1 editor.

It is also a problem for very wide includes. Those are rendered as 300px with a scrollbar.