I was very frustrated by the absence of that information - they didn't compare her to Nynaeve or anything. However, the conclusiosn I made on strength in this book (which some people don't agree with) are...

1. Cadsuane thinks that one Sister can hold Semirhage but she has three just in case. Cadsuane is probably wrong here, or Brandon is, or Semirhage is weaker than Nynaeve. One Sister cannot hold Nynaeve - the Black ajah link to shield her, and she even says that one Sister cannot hold her. Elaida insists that two Sisters hold Egwene at all times, which is ludicrous if one can hold the Forsaken. I think that three can hold Semirhage, or she would have escaped.

2. Nynaeve thinks that the Forsaken are much too powerful to be left alive. I personally thinks that Nynaeve is referring to Semirhage's superior strength here.

3. Sheriam says that the woman outside her tent has incredible strength in the Power. She remarks on it twice. Despite knowing Sharina and Nynaeve's strength, she still comments on Mesaana's, implying that it's the strongest woman she's ever felt. Mesaana may be one of the strongest channelers in the world - just below Lanfear perhaps.