Hi
Normally if an error occurs during the FtpGet, the action should stop and raise an error. If you had issues with this behaviour feel free to share them with our support team.
The FtpGet action sets the "ind_session_file_op_lst.file_length" column with the downloaded file's size (after download, locally to the runtime).
For calculating a file size with the current versions of Stambia, simply use a rhino scripting action with a code like this one:
filePath = "d:/data/datafiles/employee.txt";
file = new java.io.File(filePath);
print(file.length());
Note that the next version of Stambia will have a new feature "propertyField" on the File metadata. It will allow to get file properties directly in mappings. An article will come soon on this subject.