When you place the transistor, give the width a parameter - e.g. "tranwidth", and then define that variable's value in ADE XL to be the "mean" value of the width (11u).
Then you can create a file, stats.scs, containing :
vary tranwidth dist=unif N=9u
reference this file via Setup->Model Libraries in the test. If using corners as well, you'll need to add it into the corner definitions - alternatively add it to the Definition Files in Setup->Simulation Files in the test, and then it will appear for all runs.
Then run Monte Carlo.