My latest computer (an Ultrabook with Windows 7) had never had Opera Developer installed on it. 24 on Linux had some issues with importing bookmarks so I wasn't sure I wanted to install it on my new laptop ... but since 25 had a bookmarks manager I knew I could import them that way, so I went ahead and installed 25. Of course the window wasn't the size I would have picked, so I resized it but every time since it comes up in the same size it first did and never in the size I change it to. (Who wants a browser that only takes up half on the screen height?)
Now that I say it, I realize that I've never actually resized 25 on any of my other computers - they all had 24 or even older Developer builds on them, so they were already a size I wanted. I suppose I'll have to see if any of them remember changes to the window size. (This one does remember my bookmarks and various settings, so it's not like my profile is locked. It just never changes the saved window size.)
Have you tried toggling the window between the two states (maximize and restore) using the middle window border control icon in the top right hand corner of the window, then resizing the window to the size you wish and restarting Opera ?. Try doing it with both states and see if either saves your desired window size/position, restarting Opera each time.
Apparently 7 will maximize the window if you touch the edge of the screen or the task bar while resizing - I already had to unmaximize it a couple of times. No, it still didn't remember. Unless it's just not designed to deal with that big of a screen (1600*900, but only 14 inches). But then why is 23 fine on the same machine?
Well, on my other Windows 7 laptop here it's doing the same thing. And I don't mean remembered size, it is appearing as a window which is ... somewhere near 1000x500, might be 1024x512. Looks to be the same size (not percentage of screen - actual pixels) as on the Ultrabook. Which is strange, in that when I ran it earlier on this machine I'm sure it was larger.
Fixed as of latest update.
Ah!, so it was an Opera problem, I thought it may have been an OS problem/conflict.
On two different computers, and only 25?
Problem is not fixed in Opera 28 on Ubuntu!