There's no total size limit, though there is a 20MB limit on each individual file.
Can you give an example of where one file has overwritten another? I've never known this to happen in many years of using it!
It is possible to purposefully "overwrite" an existing asset by uploading a new revision of it, so if you've found it happening then do ensure that you are adding a new asset rather than accidentally uploading a new revision of an existing one!
Joe - Voice Admin