I saw many window stoppers online, some more sophisticated than others, but one thing I noticed with all the ones I found is that they are stiff. While at first it does not look like an issue, it can lead to damage to the window or frame if someone tries to close the window forcefully. To mitigate that, I designed this stopper to be flexible and dispensable in the case someone forgets it's there.

The model is “soft/flimsy/flexible/so on” BY INTENTION. For your window safety, don't put anything stiff to hold it open. 

If someone close the window, it may pull it out of the frame or bend it. 

There are 4 sizes available based on the distance between the frame and the window (see how to measure pictures).

25mm, 30mm, 35mm, and 35mm wide.

For most wooden double windows common in Europe, the 30 or 35mm should fit (if your window is up to 1mm smaller than the closest side, use the bigger one; it would flex into place easily). 

For New alloy windows in 99% of the times the 35mm wide will be the best option.

The 25mm is for smaller windows like toilets and showers.