Is there a way to customize the prompt so that it is bigger and stays centered on the page?

Yes we do allow you to do some additional customization to your opt-in prompt.

Here is a link where you can find out which CSS elements are used:

Customizing the Opt-In Prompt

Alternatively you can trigger the opt-in prompt with an image or a link, which we have documentation for as well:

Manually trigger the opt-in prompt