Well it's quite a broad selection of positions you're gunning for. You should also look into the roles of these positions. You may be able to knock people over but most positions should be able to do that. Are you going to be able to do that to everyone? If you aren't able to, what will you do? How good is your tackling? Can you quickly get to your feet after making a tackle? How good is your pass? How good are you at goal-kicking? How good are you at tactical kicking? Do you have a good side-step or hand-off? You will find out that choosing a position doesn't always depend on your size but also your set of skills and your teams game plan. As for indoor practice, it's always good to work on plyometrics. This will improve your power and speed. There's always good plyometrics workouts to do indoors. Apart from that just anything that improves overall fitness.
