Welcome, willoL! I'm sure someone more official will welcome you shortly too!
I can't help with absolute dimensions, but my usual bag is similar to yours (around 400 x 310 x 250mm, and I don't think 450mm would be a problem) and easily fits on its side into the wardrobe at 1K and 2K on the A380. I think it would fit in the wardrobe in all A380 First Class window seats (but I'd have to refer to someone else for the middle E/F seats as I'm not sure they're quite the same shape?).
I'd be very surprised if you didn't get help, if you needed it, too.