Does the synced blockchain always grow in size? Every time the Bitcoin-Qt is launched, the blockchain syncs to update to its latest state. Does this mean the data stored locally will always grow continously, eating up all available space eventually? Or is there a size-limit so that only the latest info is kept? How does it work?

At the time of writing, my folder is 5.82GB; let's see if this changes..
The Bitcoin.org client v0.7 and lower will continuously increase in size about once every ten minutes, on average.

Blockchain.info provides a chart showing the size of the data (actual disk consumption will be slightly larger even.)

The Bitcoin.org client v0.8, being developed now, dramatically changes how it stores and accesses data that will reduce the amount of disk storage space to some degree but also will provide a dramatic increase in performance.