Lansweeper's built-in file scanning feature, documented
here, can only retrieve the properties of files whose specific file paths you provide. Unless you have the same user account on each computer, querying the .ost file sizes with file scanning will not be possible. As user servicedesk indicated as well, these files appear to be stored in user specific folders and Lansweeper's file scanning feature does not currently include a user parameter. You would have to provide Lansweeper with the *exact* file path of each user's .ost file.
If your Lansweeper license allows it, you could potentially create a deployment package to retrieve the information and print it to a file. We cannot provide support for creating specific deployment packages, but a sample package that searches your computers for specific files can be found
here.