Is there any way to make the comments larger so that they'll be easier to read?
Topic: Comment font size?
Posted under e621 Tools and Applications
If you mean for every comment, then no, not natively on the site.
For individual comments you can use headers, but it can only be used for an entire sentence.
Go to "Account" -> "Settings" -> "Advanced" tab, and paste the following code into the "Custom CSS style" field:
article.comment .body { font-size: 120%; }
You can change the "120%" to a percentage value relative to the original font size.