How to populate paypal quantify from the form

Hi,

I would like paypal field to select the group size from the field above?

If I understood you correctly you want to pass the value from "Group Size" field to the PayPal product quantity field. 

Unfortunately, that is not possible. 

You can only perform a calculation inside of the form and pass it to a payment field. 

How to Pass a Calculation to a Payment Field

How to Perform Form Calculation Using a Widget

Feel free to contact us if you have any other questions.