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.