This is happening with Winamp 5.0 Final, yes
Not one of the pre-release beta/rc versions?
Are the relevant settings enabled?
Prefs (Ctrl+P) -> Video
"Resize video to fit window" is the main relevant setting.
It would probably be easier for you to list which other options in there are NOT checkmarked (fullscreen settings in middle aren't relevant to the case though), mainly just the top 4 settings, and the bottom 5 ones.
Apart from that....
Are you using the modern or classic skin?
Does it happen in both?
Or are you using a 3rd-party skin?
And, if it's the default 'modern' skin
is the video window attached or detached?
Now, I vaguely remember someone else was having this problem in one of the earlier rc versions, and the search results for "Resize video to fit window" are pointing to this
but it's a thread with 609 posts
so it's gonna take some time to find the relevant posts in there...
Bear with me...
...ok, here they are:
Problem occurs when "scaling"
in main (if vid attached) or video (if detached) window
is set to "locked".
Checkmark "temporary" instead
(window -> main menu -> window settings -> scaling)