I've been saying the Spurs need more size long before it was popular opinion and they're still not overly big, but with Splitter signed and once Jefferson is officially signed, they'll be big enough. They still don't have a 3/4 tweener, but this notion that they need a 6-7 SF to compete for a championship is a myth.
The Celtics played 6-4 T. Allen as their backup SF (and SG), the Lakers frequently played 6-6 Bryant as their backup SF, the Magic played 6-6 Pietrus as their backup SF, the Nuggets primarily played 6-5 Afflalo or 6-6 Smith as their backup SF, etc. All of these players are natural SG's. Not one is the size of a true SF, yet they were all successful teams to varying degrees and the size or lack thereof of their backup SF was not an impediment.
Basically, you need one legit sized SF (Jefferson fits the bill), but you can get away with Bell or even Hairston (size-wise) playing backup SF. The big thing is, can whoever it is filling this role play above average individual defense and shoot the three at a high percentage, not whether the guy is 6-7 225 or 6-5 220.