Luke might have asked his dad?
Or Bruce could have anticipated that someone was going to show up and there's 15 Batsuits out the back cut to the specific specifications of whoever turned up.
Which makes Luke a conman, an actor, or a stage magician, or all three, to roll with it depending on which character broke into the Batcave.
And also means that Bruce would have a shortlist of People He Knows As Most Likely to Figure Things Out and Try to Pick Up Where He Left Off to have suits pre-measured for.