Run a File Size Report
This won't list everything, but instead quickly shows you the largest assets stored in your Asset Bank.
As an administrator, navigate to Admin > Reports > Asset tab.
Under All Items by Date and/or File Size build the report by specifying a date range by upload or modification date, and a threshold file size (for example, files over 10MB in size).
Click Get Report to run the report.
At the bottom of the results page, you can download a list of the assets to an Excel spreadsheet.
Wild Card Search and metadata export
Using this method, you can list every asset in your Asset Bank in an exported Excel file with all metadata (including the file size) detailed.
In the search bar on the homepage, search for an asterisk. This is a 'wild card' and will return all assets.
Run the search.
On the results page, scroll down to the bottom and select Export all results...
Ensure the 'Export asset files' is NOT selected, and that the Export as native Excel 2007 format checkbox IS selected. Then, click Run Export.