48 MB is OK for "dalvik.vm.heapgrowthlimit" which is used for Apps that doesn't require the android:largeHeap="true"
So for Apps that require the android:largeHeap="true"
(like YouTube, Browser App, Maps and so on) the build.prop property "dalvik.vm.heapsize" is used, which i recommend to be not lower than 128 MB.
Usually it's best to keep such Values to the lowest possible in order to save RAM.
BUT .. if they are too low, the Java Code Processing to the Machine Code compilation will run in a loop, (garbage collection) resulting in higher cpu load = higher battery draining.
if u don't declare "dalvik.vm.heapgrowthlimit" then this Value will be automatically the half from "dalvik.vm.heapsize", according to the Source Code.
One more useful Tip:
don't forget to keep the "m" at the end of the Value ... otherwise the Phone wont't boot anymore.
Anyway, if u set a too low value and u don't want to restore a cwm backup or to flash the CM11 package, then simply boot to cwm and connect the Phone with your PC ... run ADB and type:
The full build.prop will appear on your PC and u can EDIT every single line .. when u're done, exit with "Strg" + "X" and save with "Strg" + "Y" + ENTER. then it should boot, once u fixed the wrong lines.
full latest build.prop: (optimized)