Leonard and Diaw are actually pretty close in height within a couple of inches of each other while Leonard is much longer. Diaw does have 20-30lbs on Leonard, which does keep Dirk from backing him down. I'm not so sure Dirk would be able to take advantage of Leonard given the help defense. While Dirk can simply shoot over Leonard, that's really true on any player that the Spurs throw on him. He will get his points but I don't see him torching Leonard late.
I could definitely see Pop using a smaller lineup given its mismatch potential on offense. Pop historically; uses smaller players to crowd Dirk when he faces up (Bowen and Ginobili); having multiple player defend Dirk and using Duncan to rotate over if Dirk posting up or catching the ball over a fronting defender.
Splitter and Diaw are going to be the main options, but I'd expect Kawhi to see crunch time minutes in a couple of those games at the 4. That is unless Dirk gets hot and Pop goes back with Diaw at the 4 and slides Leonard back down to the 3.
If the Spurs go small, Dirk would either have to cover Leonard, Manu, or possibly Green in that situation (assuming Marion would be tasked with guarding Parker or Ginobili).
Mavs closing unit against Spurs Small ball unit
Calderon-Parker
Ellis-Green
Marion-Manu
Dirk-Diaw
Dalembert-Duncan
Potential cross match ups
Marion-Parker
Ellis-Green
Dirk-Manu
Calderon-Leonard
Dalembert-Duncan
that would leave the Mavs vulnerable to post ups from Kawhi not to mention the potential for offensive rebounds for the Spurs.