Which winter coat should I buy? I'm indecisive and need your help!

I've been looking and dreaming at knee-length, wool, winter coats for months now. I've been wanting a classic one for years now and finally have the money for it. I'm trying to decide between a slightly large/loose coat and a 'fits like a glove' smaller one. I'm able to wear lots of layers (and move!!) with both coats. I want the smaller one, but my Dad is warning me that I'm going to gain weight/get fat (I'm 24 and though I've fluctuated, my base weight's been the same for 5 years now).

So 2x, should I buy the bigger coat that's just not as polished looking allowing for weight gain? (I want my coat to last that long). Or should I buy the perfect fit I love and possibly not be able to wear it?