What is the chain gauge for current Stihl chainsaws ( width of bar slot)

There is no one gauge that is common to all new Stihl saws. Gauge is completely dependent on the bar chosen.

The standards are 0.043, 0.050, 0.053. 0.058, 0.063, 0.080.

You should be able to find your chain gauge on the bar. Even if the paint is worn away, you should see the impression from the stamping. If all else fails, take the chain into a Stihl shop and they will have no problem finding what you need.

In the US of A 0.050 is most common, 0.053 is most common in Australia. It can be any of the sizes Roland stated. Using the wrong one will bind, make for bad cuts, and may be dangerous. HTH
Lou