I think the question people have is will Austin be able to consistently shake his man at the LOS to get downfield and be open enough to catch a long pass. He isn't going to go up and out-muscle a defender. I think he can do it, and an emphasis on running will help, but that's a legit question.
For me this isn't as simple as having Austin "get downfield and be open enough to catch a long pass"..... Unless he just blows by people, and as you said, he isn't going to win many contested balls in the air due to his lack of size. And the issue with that, is MOST teams play primarily zone coverage against the Rams, which means Austin will rarely find himself without a deep safety over the top providing help.
In his one real breakout game as a receiver, for whatever reason, Indianapolis played straight man to man. He, as well as Givens, killed them on crossing routes, and Austin even got behind of CB who he beat badly off the LOS. But these opportunities are few and far between.
The zone also neutralizes his speed and quickness on underneath stuff, as it forces him to settle into the soft spots (which is something to this point, has been difficult for him) making the few catches while at a stand still. They can talk all they want about making a concerted effort to get him more involved, but at some point the onus has to be on the player to make adjustments.
They envisioned Tavon Austin to be an upgrade over Danny Amendola as a threat in the slot. Logically, it was a sound thought. He is quicker, faster and was thought to be more elusive. The problem as I see it, is two fold. Austin hasn't shown the ability to make the cuts in tight speaces, that Amendola made, and his hands are suspect at best when in traffic. That was Amendola's biggest asset. He caught everything thrown in his direction.
What I have seen, is Austin is very dangerous when he is on the move. His quickness and elusiveness at full speed is unmatched. The problem is, he hasn't shown the ability to consistantly diagnose coverages, or be explosive when starting and stopping.
For him to be successful in this offense, IMO, they will need big contributions from Quick and Britt, showing they can occupy coverages on the outside, on the intermediate routes (15-18 yards), combined with a dominant rushing attack that forces teams to play an 8 man box. Forcing LBs to honor the running game, freezing them with Play-action, should in theory open up lanes BEHIND the LBs for Austin to find space on the crossing routes. Thereby, allowing him to catch the ball on the move, and not settling in has been the case in the past.
I just don't think he will ever be the type of WR who is capable of creating his own space. He just isn't physical enough to win one on one battles for the ball, and to think that he is capable of just "running deep" isn't gonna get it done. Sure, he may pop a quick slant from time to time, if he hits a seam, and they catch the defense in an all out blitz. But again, most teams will roll safety help to his side when he is on the field.