Charlying, that IS a huge topic. Music is a enormous part of the "Disney experience".
If I was you, Id start by picking which venue the music is used in (such as theme parks, movies, records, television scores) then work from there.
Personally, I think the Theme park music would be the most interesting because it plays such an important part in all the shows and attractions in creating a mood. Plus the composers are the same ones from other Disney Projects (Buddy Baker, the Shermans, even Danny Elfman contributed the score to Haunted Mansion Holiday!)
Although now that I think about it...even THAT is pretty huge subject. On second thought do whatever the hell you want, if you are in the position to write a thesis, you are smart...you'll figure something out.