PayPal: Is it possible to cutomize the payment field?

It's possible to customize the payment field with some CSS code, you could use this one and just add the desired rules (text size, color, field's size):

[data-type="control_paypal"] input[name*="myProducts"]{

   /* CSS rules go here*/  

}

This guide will help you injecting the code to your form: https://www.jotform.com/help/117-How-to-Inject-Custom-CSS-Codes 

I hope this helps. 

Your code is correct based on my tests, the only difference is that the "PayPal" word needs to be in lowercase.

[data-type="control_paypal"] input[name*="paypal"]

{

   text-align : center !IMPORTANT;    

color : #349955 !IMPORTANT;    

font-weight : bold !IMPORTANT;

    width:90px !IMPORTANT;

}

Injecting this code should format your field: 

I hope this helps.

Yes, it's possible to add the dollar sign or any other symbol by injecting this code: 

[data-type="control_paypal"] .form-sub-label-container:before{

    content: "$";

    position: absolute;

    margin-left: 22px;

    margin-top: 6px;

    color : #349955 !IMPORTANT;    

    font-weight : bold !IMPORTANT;

}

If you need a different currency sign to appear instead, please simply replace the current one in the "content" property. 

I hope this helps.