I like
Maximilian. I would use the nn
Max and having a sibset with shorter names wouldn't be a problem at all.
Personally I always picture
Maximilian to be paired up with some ancient classics. That is always what comes to mind when I think of
Maximilian. Like
Maximilian Augustus or
Auguste.
Maximilian Alexander,
Maximilian Cassius,
Maximilian Constantine,
Maximilian Octavius. However, many classics would go good with it too in my opinion. Such as,
Maximilian Ambrose,
Maximilian Henry,
Maximilian Levi.