Think about it this way- if it's too tight to be comfortable, it'll suck. That means you wanna be able to move a bit, and keep circulation even after the third hour on the bike. If it's too loose to be comfortable, that'll suck too- if you can grab a fistful of leather, it's too loose. It'll flap, and what's worse, it might catch on a get-off, and then you're tumbling, not sliding. Bad news.
My rule for my jackets is that they can't slide around across my shoulders, and the sleeves can't easily twist too much. That's tight enough. They gotta be comfortable with my warmest clothes I'm gonna wear under them, and I still have to be able to fold my arms and take a deep breath. Remember, leather stretches a little bit, and breaks in, so don't worry about a stiff elbow where the armor just needs to stretch the elbow a quarter inch, but DO worry if it ever cuts off circulation, or you catch yourself walking like the Michelin Man because you can't more naturally.