It's like the blazer equivalent of the boyfriend cardigan. So yes, longer and looser. You want to look like you nonchalantly threw on your boyfriend's blazer. You are supposed to roll up the sleeves so the best blazers have interesting linings like white with a striped pattern. Preferably you leave the jacket open.
This look comes in and out of style. In the 80s we wore the look with huge shoulder pads and skinny lapels, and came in a myriad of colors and materials.
I think the best investment boyfriend blazer - one that will work for decades - is to find one that fits you really well in the shoulders, but is loose elsewhere. A slight nip in the waist is good, but you don't want it to be too form-fitting. For example I tried on a Theory "boyfriend" blazer recently and I felt it was too form-fitting and too short to fit the bill. To me it was just a classic blazer.
As for ideal length, I'm thinking to the crotch point. I would go with navy, gray or black, perhaps with a pinstripe, in a 3 season wool - the classic menswear blazer colors.
I think I remember your blazer with flower pin ... I may be misremembering, but if I'm not, it's pretty close. What is the lining like? To me, that is the key element, having that interesting lining to show when the sleeves are rolled.