I'm not convinced Roko is any bigger now than Bojan is. Bojan is listed as 6'8", somehow, in the new barefoot height era of the NBA; Roko is 6'7.5". Roko's wingspan is 6'11", nothing special for his position. Bojan's is unknown, though just guessing maybe an inch or two less than Roko? Their weight these days is similar, though Roko could fill out; so is their athleticism, I'd say. Although I'd expect a little more strength/toughness out of Roko by the time he fills out, I have real questions whether he'll turn out to have any more defensive lateral mobility than Bojan does.
And, of course, his shot is nowhere near what Bojan's ever has been (who himself was a second-rounder).
For those on the length/athleticism train, you should be all over Greg Brown rather than Roko.