Is it possible to get a higher res thumbnail of the customized product?

I am doing a custom design in the cart which requires the product image to be displayed at a bigger size. But the thumbnail generated by the plugin is generally very small, thus making it very pixelated.

What can I do to increase that size?

Yes, it's possible to increase the size of a customized thumbnail. You can do it by visiting FPD settings > Woocomemrce > Cart. There, you will find a field to submit size in pixel for the thumbnail.