size of wrench for oil plug?

I think you would do better with a 1/2" socket wrench and a set of 6-point metric sockets. That will give you better leverage and you will have the option to use a cheater bar if the oil plug was over-torqued.

I don't recall if the size required is 14 mm or some other size, so hopefully someone else will respond with that data.

 

Here is the thing about working on cars: if you have only one tool for the job, you will inevitably run into trouble that requires some other tool. If the Prius is the only car you have access to and it is raised up on jackstands needing for you to fix it before it can be driven, you will be in trouble.

So it is better to have a variety of tools so that you have some chance of recovering when you run into a problem.

 

You are correct Patrick it is 14-mm.
I always use John1701a's pdf instructions, not sure if John still has those posted on his personal web page, but its fantastic.

Box end wrench...not sure on that...I use a Craftsman regular socket ratchet wench and an extender.