From LimeSurvey Manual
Maximum file size allowed (max_filesize)
The maximum size of a file that should be allowed to be uploaded by the user.
If the maximum filesize is set to 1024 KB, any files larger than 1024 KB will be discarded, and the user will be asked to upload a smaller file.
Note: The final value shown to the survey participant is also depending on PHP settings, like post_max_size and upload_max_filesize. Of all three possible values (upload_max_filesize, post_max_size and max_filesize) the lowest one will be shown and used.