High verses medium verses low density is all about the BTU's per square inch being delivered by the element. The lower the BTU's the lower the risk of scorching. It's kind of like the difference between putting a steak on a searing hot grill verses a hot grill. The lower density the better when it comes to brewing and even more important with mashing.
So, larger is better for the same wattage because if you double the size the same heat is dispersed across twice the surface area - it's a lower density element.
Concerning copper verses stainless - the yeast cells need a little copper anyways. And I've seen may brew kettles made of copper over the years. Copper won't hurt you because it's a relatively safe metal. You ingest minute amounts of copper from the change in your pocket every day.