Is this unit new or refurbished?

It was a gift for someone, but I believe it was brand new. They are very happy with it.

What is the size of the interior?

max. depth = 10" max. width = 10.375 max. usable height , between top of lowest tray and top element = 5" I've had about a year, pretty much use it for toast and heating leftovers, it works well, simple, small, heats up fast and temperture matches what the oven thermometer says it is should be. Nothing fancy about it, I'd recommend it.