Dikembe Mutombo -- Hilarious Rental Car Fail ... I'm Too Damn Big!!

File under: "The Perils of Being 7-feet Tall" ... NBA legend Dikembe Mutombo had major trouble getting himself a rental car in D.C. this week ... because he didn't FIT in any of the rides!!!

You gotta watch the clip ... Dikembe was optimistic at first ... but quickly gets shot down at TWO DIFFERENT rental car desks that didn't have any whips big enough for the 7'2", 260-pounder.

Finally, Mutombo gets his hopes up at Avis ... but when he tries to get in the tiny Ford Taurus they tried to give him -- everyone realizes it ain't gonna happen.