Wasn't the plant they bought used to make around 6K cars a week when it was for Toyota? Tesla has plenty of space to ramp production, they are only hampered by lack of funds. 400 cars/week should not be a problem once they get seriously going.
They should already be at 400 cars a week and I'm sure they want to get it up to 500. Size of the plant is not the issue. The number of robots, what they do, and how long it takes for them to do each step is important. Then there are certain things that only a human being can do, such as thread wiring from one end of the car to the other. All of it takes time. Soon they will be focusing on removing errors from the process so they can go even faster.