I can't decide which winter wool coat to buy and need your advice (size and color).

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. It's come down to two choices--a black or a green. The color is less of an issue (I have auburn hair and green eyes so the green might be the one I choose) but the fit is the problem. I'm trying to decide between a slightly large/loose 6 and a 'fits like a glove' 4. I'm able to wear lots of layers (and move!!) with both coats. I want the 4, 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 6 that's just not as polished looking, JUST IN CASE I gain weight in the next 5 years? (I want my coat to last that long). Or should I buy the perfect fit, absolutely classy 4 I love and possibly gain weight and not be able to wear it? I'm frustrated thinking about it :( And here's the coat I want: http://www.jcrew.com/womens_feature/catalogjcrewcomexclusives/jacketsouterwear/PRDOVR~49622/99102451340/ENE~1+2+3+22+4294967294+20~~~0~15~all~mode+matchallany~~~~~lady%20day/49622.jsp