Hi Imy,
Sorry I saw your message but was getting ready for my SA3 exam last week and forgot to reply until now.
I cannot remember how long in words it was, though it was quite detailed.
For my mentor to write the letter I gave him a detailed list of the types of models and statistics I used in the dissertation (I also gave him a copy of the dissertation.)
I also looked through actuarial papers using the IFoA webside and documented why the techniques which I had used are relevant in actuarial work.
My msc was in statistics, my dissertation was using health data. But many of the models and techniques I had used are used in the world of actuaries. Such as GLM models and bootstrapping to name a few, I also used R programming which I made sure I got across as the institute seems keen on this now. This formed the basis of my mentors letter.
Hope this helps