I realize this is an old topic but I am responding to it because it comes up on Google searches. For anyone who is getting the "not enough space on /system" error when trying to install Avast Anti-theft, it is likely true. As of Android 5.1.1 there was not enough space left in /system, possibly because Google added bloatware. Ccarefully deleting some unnecessary items using Titanium backup or manually will free up enough space:
****I take no responsibility for this information and any action you take is at your own risk****
Here is what I uninstalled using Titanium Backup (with root of course...):
- Google Fit (icon is some sort of multi-colored heart)
- Google Hindi Input
- Google Pinyin Input
- Google Korean Input
- iWnn IME
While I did not uninstall these, my research indicates you could also uninstall Email and Exchange Services (NOT Gmail!!!) if you don't use them.
There are likely many more things you can delete such as unused ringtones. Really you could delete all but one and copy them into a media folder on your SD card and just point to them, no sense in them taking up limited real estate in /system. This space limitation was at best very short-sided of Google, at worst intentional to discourage rooting.
I hope this works for you.
Re: not enough space available on /system