the class for it is "gform_confirmation_message". There is nothing for the sizing of it. Add these, I stole from the paragraph text.
https://stmarysgi.diocesanweb.org/styles/test/
There is a small untitled single line text form, for you to test on. I added the fix in tweaks for now, but would be good to have in the theme.

    font-size: 18px;
    line-height: calc(5px + 18px);
    font-family: 'Montserrat', sans-serif;