Not sure if I understood your question correctly, but in the codex it reads:
If adding the excerpt manually, you may use (some) HTML formatting and
the tags will not be stripped.
So if the only thing you want to do is make part of the text bigger, just add for instance:
example text <span class="bigger-text">example bigger text</span> some more normal text
And make font-size of .bigger-text larger than the normal.