Generally I wouldn't recommend using a larger bar than is recommended. I think the biggest issue is you will burn out your saw.
However, there is one thing to think on that might make it more 'ok'. When milling with a chainsaw there are special chains (ripping chains) and bars you can use to reduce the strain on the saw. Like other blade, most chain saw chains are designed to cut across the grain, and when you use those to cut along the grain it takes more work for you and the saw.
My recommendation would be to talk with a stihl dealer and find out if you can go longer if you use the bar and chains for milling.
Found the following information about the mills you are asking about
The general rule is, the more power your saw engine has, the faster the cutting speed. Almost any engine that runs will cut, it just depends on how much time you want to spend milling your lumber.
General Guide for Chainsaw Power
Log Size - Engine Size
- up to 18" 55cc to 67cc
- 18" to 36" 68cc to 85cc
- 36" & larger 86cc to 120cc