I play Diablo II using wine and it works quite well. The problem it that I would like to play in windowed mode and 800x600 tiny window looks bad on my 1680x1050 display. So question is if it is possible to zoom (enlarge) only one window using kwin (accelerated by gpu). All zooming effects I found in KDE zoom either whole desktop or only a small area around cursor.

No that is not possible and I doubt it would solve your problem, as it would still look bad (just in another way). To make that work, we would need Input Redirection

Too bad

Actually, it would solve the problem. I dont want it to look perfect (the game is 10 years old), just make it a little bit bigger. The game doesnt look that bad in fullscreen and fullscreen is kind of "zoom" as well.

The zoom effect included in kde, which enlarges the whole desktop, is almost what I want. The problem is that when I move with the cursor, the enlarged area is also moving which makes it unusable for playing games like Diablo II. I would prefer something like "active edges" - the enlarged area would move only if I reached an edge of the area with the cursor.