Is there an 'upper limit' to the size of the cache in Version 25?
I tend to keep all the stuff listed in the 'Clear Browsing Data' window. People smarter than me have told me to clean out my browser 'caching', in particular........ 'frequently' [whatever that is].
Any actual FACTs on this topic?
I would imagine the size limit of the cache in Opera 25 is only limited by the disk space available, unless there is an unknown built-in limit.
You can't change it on the settings interface as you could in Opera 12.
Clearing browser data is purely a personal choice as to whether you want anyone else to be able to see where and what you've been surfing.
If you are the sole user of your computer, as I am, I don't see it as being an issue.
Of course there have been many incidents of people being nabbed for illegal activities after having had their computers seized, or even when the machine has failed and been taken in for repair, because people have seen what's in the browser history!
I would hope you've nothing to fear on that count!