Apparently he measured at 8' 9.5" and weighed 228 at a summer testing at P3 training center in California.
Is Luka Dončić 2018 NBA draft 1st pick?
He has a 8'9 1/2 Standing Reach
Here's P3 comments on him based on 2016 summer measurements/testing:
Luka has spent a few weeks at P3 the past two summers. Luka combines outstanding anthropometric measurements, including an 8'9.5" standing reach (88th percentile for NBA guards) and 228.8 lbs of mass with very competitive and improving movement qualities. In the lateral plane, Luka's ability to abduct the hip (1 dev above the mean) allows him to generate Force more efficiently than most NBA players that we've assessed (71st percentile in lateral acceleration). He's become particularly adept in a change-of-direction environment, where his Slide Agility times outpace most NBA Guards tested at P3 and they've improved by roughly 0.25s in the past year. Vertically, Luka ranks better than 73% of NBA guards in height touched during the approach and his improved his countermovement jump by 2 inches in the past year. He's good now, he's still getting better, and he's well within his physical development window!
https://forums.realgm.com/boards/viewtopic.php?t=1584464#p58586672