Fetch records from a sub form and fill dropdown field

Fetch records from a sub form and fill dropdown field

I have a Product form with Product and Price for various sizes. Please see the attachment pic1.png.

I have a estimate form to select the Product. Once I choose the product, it should show available sizes in the dropdown.  When size is selected, it should calculate the cost of the product. Please see the attachment pic2.png.

1. How do I fill the dropdown the sizes based on the product selected?
2. How do I get the price when the size selected?