Use the "search" function, to search on a simple word, such as "the". Don't worry if you get "zero results".
Then, down the left, under "Advanced Filters", there is a "Size" option. Click on "Size", and choose "larger than 100 Kilobytes. Don't worry if you again get "zero results".
You will see something like:
Click on the "X" to the right of the word "the", to rerun the modified search.
What you should see is a list of all "large" messages in ALL folders.