Hi,
I’ve looked at the page source through the dev tools console, and it looks like the rev slider is changing the font-size via a forced style attribute. I guess it is some sort of fix for another widget or something similar.
To override that feature, use this custom CSS:
[html].probox input {
font-size: 12px !important;
}[/html]
This will force the original 12px height on the input fonts. Apply this code to your theme custom CSS field (if it supports it) or use the custom CSS field on the search plugin back-end.