I have a couple of 66" bows, and they are among the smoothest shooting bows I own. Under the right conditions a 66" bow is as good for hunting as any length of bow made, and better than most. A longer bow is always more stable and smoother than an otherwise identical shorter bow. A higher moment of inertia and longer limbs that bend at less angle for the same draw length are physical attributes that cannot be totally compensated for by other means, just like a larger objective lens on a telescope is inherently desirable. That said, I never choose one of my 66" bows for hunting because they do not have the versatility and adaptability of my shorter bows.
For most of my hunting today, I generally pick up one of my bows of 60" and less that is designed to function well at my 29.5" draw length. For carrying around, shooting from a tree stand or ground blind, transporting in a vehicle, and using in other restricted areas, the shorter length and lighter physical weight of my 54" to 60" bows far more than makes up for the advantages of a longer bow.
Even though I make those statements about shorter bows, until the last few years when I discovered the newer bow designs that could result in really good, short lengths, most of my hunting was done with 64" bows, which are not that much shorter than a 66" bow. Of course, I am 6'1" tall, so a 66" bow is not as comparatively long for me as it is for you. I used my 64" bows in lots of different hunting conditions and took quite a bit of game with them without ever thinking they were too long. And the utility of those 64" bows has not changed over the years. They are just as good for hunting today as they were 5, 10, 20, or 30 years ago when I was shooting them all the time. In fact, if I didn't have my shorter bows, I would gladly hunt with a 64" or 66" bow all season long, every season, for the rest of my life.
Back when I was really young, as in ages 5 through 25, I shot shorter bows of older designs. Of course, that was no problem when I was 14 or younger, but as I started growing much taller and increasing my draw length between 15 and 19 years old, bows like my old Browning Nomad II started stacking on me, pinching my fingers and feeling less than ideal. A 64" bow was a welcome change for me when I finally got one and had a bow that was smooth and forgiving and shot well for me.
Did you make a huge mistake? I would say the bow is definitely big (maybe even huge for someone your size), but the mistake is probably a little one. The bow will work well for you; however, if you were to shoot a really nice, shorter length bow, you would probably wish you had ordered a bow like that rather than the one you did. No problem. Learn to shoot well with the bow you get and then try out a bunch of bows next time before you buy your next one so that you won't have to ask yourself and others whether you made a mistake.
Allan