This would depend on how stringent you need to be with space on your device. If you're running a lot of stuff on your device and don't have that kind of memory free, then lower the cache settings to help conserve space. 5000 is not out of the ballpark.
Another consideration is how often you read the content and how long you need to store it. I read mine daily and don't really need to store the data over a long period of time, so the main article cache is set to 2000 -- but I have 16+ megs free. I could raise that number but 2000 works in my case for my style of use.