Increasing label size for "Authorize.net" plugin

I am creating a form using the "authorize.net" plugin and the field for the payment amount is really small. Is there a way to increase the size so it shows more of the price? I have tried changing the "size" in the source code but it does not seem to work.

Thank you

To adjust the size of the payment amount field, you can try injecting custom CSS on your from. 

For example:

#input_25_donation {

    width: 80px;

}

Here's a guide on How to Inject Custom CSS Codes.

Hope this helps.
Regards.