I don't believe it's possible to go so granular - just QVS as a whole rather than each Document. If you've got a clustered environment you could perhaps isolate documents and monitor their individual usage.
Read these documents about using Windows Perfmon to get detailed metrics about Performance:
And what with QlikView Optimizer application? It uses .mem files and there are some size values of each object in document, but I cant find out, if the sum of these values is the size of document in memory and if these files could be generated automatically. Does somebody have experience with .mem files?
The size of a document in Memory can be roughly calculated by multiplying the size of document on disk by 4.25.
The size of a document in Memory can be measured by isolating the system then opening a document at the same time as monitoring the memory growth in task manager.
Neither of these take in to account cached results etc from user activity.