Buying A Mac Coat

Hi all, recently I've been considering getting a mac, specifically this one from ASOS.

ASOS | ASOS Twill Mac at ASOS

But I'm not sure how to style it? Does plain jeans and trainers work with it? Can it only be worn smart and not casual? Can you even just wear it open (doesn't always have to be buttoned up?). I know it's a lot of questions, but if anyone could give me some guidance it'd be much appreciated!

Thanks

I find it can work with anything, either way: undone, done up, collar down, collar up; however you like to wear it or feel most comfortable.
It can be dressed up or down (really depending on what's worn underneath).

So it's pretty versatile then. Good to know!

Be wary of colours, I've found colours similar to the jacket do not go well, it creates a deconstructed suit look, which in my opinion looks terrible.
As Ben said, it can be dressed up or down, wearing it open gives a more casual vibe.

One of my main worries though is it may be a bit big for me? Is it worth consulting a tailor about it? Or am I going overboard slighty?