According to the Android reference page, FileBackupHelper should "be used only with small configuration files, not large binary files."
I managed to successfully backup a database file of 8 kilobytes, but I'm curious at what point a file becomes a "large" binary file. Do you think Android is referring to files in the megabyte range?
I believe there is no hardcoded limit in Android source code (which defines "large" vs "small"). At least, I didn't find within 10 minutes looking through Android source.
Generally speaking, you should take into consideration a speed of wireless connection. It depends a lot on where you and your users are located. If it's major cities with 3G/4G coverage - I think you can backup a megabyte, if's rural area and your users are charged per kilobyte, I would rather scale back to dozens of kilobytes.