It's that wide probably because one of the entries (one out of the view) is that wide.

At least you can see the file mask, but does the window really need to be that wide?

It will still fill the page from edge to edge as you can see in the attached picture.

In other applications the drop down menu has the same width as the row i click on.

Thanks for your report. I hope this is fixed in the latest beta version 6.6.
https://winscp.net/eng/downloads.php#beta
https://winscp.net/eng/docs/history?a=6.6

Bug fix: Too long edit history dropdown can overflow monitor bounds.

Can you please test it?

This doesn't look right when i open this drop down menu in file color.