Getting Venners if you are a dentist or dental assistant please help?

I'm about to get veneers they are being made right now. I 'm wondering if when they are delivered and I go to the dentist to get them if perhaps I want to make them longer will I be able to do that? I'm afraid the front teeth won't be long enough? But once they are made will I be able to have them add lenght?? Thanks so much.