How can I increase the product image thumbnail size in a purchase order? It seems to be set at 50px.

You can do that using injected CSS code like this..

.form-product-item img { height:100px; width:100px; }

You can check similar discussion on this thread.

Actually a much better feature I can recommend is to use the Product Lightbox Widget.

It has a lightbox of the product image when clicked.

Here is a demo jotform you can test http://form.jotform.co/form/33604136500846? where the product lightbox was used.

Let us know if you need further assistance.

Thanks.