it really depends on your data but QV is very very good at compression data so it is not rare to see 7-10 to 1 compression ratio. With my projects it was close to 10:1 a lot of times.
What you should worry though is not QVD size but the amount of RAM your final QVW will consume. Also if you will be using AccessPoint/QVS, additional users will consumer 5-10% of RAM.
I think there is a white paper about this in Qlik Scalability space. Sign up - it has a lot of good info!
When you extract any data source file it stores as qvd. The qlikview stores data In QVD In encrypted form. So data are compressed In QVD.
If a data source file is of 10 MB size. When its convert into QVD then size reduces up to 90%. So in QVD size of data is near about 1 MB.