As with all things there is a limit in the exporter. The limit of exporting data is determined by the width en length of that what you are exporting. The with is determined by the amount of columns and the length by the amount of records you are exporting. The export will create a list of records (to be exported) in memory, so memory is the limiting factor. Writing the actual file will be done record by record so no memory limits there. I haven't seen the limit yet and the limit depends on the environment's available memory. If you run into any limitations just drop me an email and I'll try to be of assistance.
Yes, you can schedule exports. You probably need something like the SFTP module to transfer the documents to an external source. But the scheduled event will create the csv files as filedocument objects in Mendix. As to the limitation I have no answer to that. Erwin 't Hoen wil know the answer to that one. But since it is csv I think it can be quite large.