@Ayan, let me give you several pointers. They all allow to find the list of files (with their size), but each time this will be for one single package at a time (a package corresponds to a venue). You will have to repeat this for all packages you are interested in.
In the GUI, from the main menu you can go to Standard Solutions - Available Files. Select TRTH Venue by Day from the drop down menu. The list of packages is displayed. Click on a package to see the list of files(with their size). As far as I know it is not possible to export the list of all files for all packages.
If you are using C# you can look at the .Net SDK Tutorial 2: Retrieving VBD files (note that the code installation must also be done, it is described here). How to retrieve the list of files (with their size) is described in this step.
It would be fairly easy, based on the code from the samples, to create a list of all the files with their size.
For Web GUI, you can click the DATASCOPE SELECT menu on the top left corner and then select "Available Files"
For REST API, you must first list all available venues by requesting list user packages.
Each venue will have a package id which you can use to get a list of files present under that venue, along with the file size.
Alternatively, you can use the GetUserPackageDeliveriesByDateRange method to retrieve list of files by date range.
GET https://hosted.datascopeapi.reuters.com/RestApi/v1/StandardExtractions/UserPackageDeliveryGetUserPackageDeliveriesByDateRange(SubscriptionId='0x0400dc1d24a00cb4',FromDate=2016-11-22T20:54:03.671Z,ToDate=2016-11-24T20:54:03.671Z) HTTP/1.1 Authorization: Token <your_auth_token_goes_here> Prefer: respond-async