There is no specific size or formula that suits all applications, it's up to you to analyze your usage and come up with a suitable solution. A cache that is too small causes additional overhead with no benefit, a cache that is too large can once again cause java.lang.OutOfMemory exceptions and leave the rest of your app little memory to work with.
http://android-er.blogspot.com/2012/07/lrucache-with-different-size.html
