Unless you are resizing the window later, that above code should have just proved they are sized correctly.
Another thing you can try is put the GetWindowRect confirmation somewhere where it can be triggered. So after you see the window, when you press 'space' or something, check the window size again and confirm it is still correct.
If it's correct, it's your eyes playing tricks on you.
If it's incorrect, you are resizing the window somewhere, and that's what you need to find and fix.
On the off chance that this is the case.. Henri, are you sizing the window with the expectation of the client window being 800x600? If that's the case the window size is based on the entire window, not the client window.
The client window is the space within the border of your entire window. When you create a window and give it a width and height, this includes the border, title bar, etc. not just the space within the border.
There's a function called GetClientRect which returns the current size of the client window. You can use that with GetWindowRect to re-adjust the window size accordingly after it's been created. I can give you a function that does this but will only do so if you ask for it incase you'd rather do the problem solving yourself.
Looks like the issue is more likely to be what Disch pointed out. Are you absolutely sure that _m_uiSize == 800, and _m_uiSize == 600 ?
Thank you, Disch! I was already aware of this prefix and posfix feature of decreasement and increasement operator and what's the difference between the two, so I'm surprised I didn't notice that kind of mistake in my code. Thanks for pointing it out, Disch!