If you feel like the length of your backswing is causing an outside to in swing path, it is probably because you are losing your spine angle at the top of your swing due to lack of flexibility. This almost always gets you across the line and causes an out to in path. Try to focus on your spine angle and stop your backswing when you start to feel it change.
I am tall and have a very long swing, but fortunately I am also still pretty flexible. I've tried to shorten it over the years, but it doesn't really provide me any improvement so I just go with the long swing.