If you intend to buy: Contrary to a popular idea, the smaller the courgette, the better it is. As they grow, courgettes become full of water and lose both taste and crunch. The famous chef Alain Dutournier says that "Over 15 cm (6 inches) a courgette is no longer of any interest".
So big or even huge courgettes are not better than small ones, but are only good for being stuffed .
By "mash powder" do you mean mash potato powder?
If so, yes you can, see how to on this chicken recipe : chicken breasts in a potato crust.
Can I use potentiometer mash powder to make cutlets?