512x512, I think
And you can look up the paths with an MPQ viewer/editor, but here we go...
UI\Glues\ScoreScreen\ScoreScreen-NightElfVictory\ScoreScreen-NightElfVictory.blp
UI\Glues\ScoreScreen\ScoreScreen-NightElfVictoryExpansion\ScoreScreen-NightElfVictoryExpansion.blp
UI\Glues\ScoreScreen\ScoreScreen-Defeat\ScoreScreen-Defeat.blp
It appears there's no racial defeat screens.