I’m not a pro but I think I can tackle your questions Vegeta.
Too big?: You can make your screen as large as you want. You will have to wait while it fills which is the factor there.
Plants position?: this Isn’t a huge factor, but you want to be able to access them. Give yourself some room to work. The plants branches can move into place as they grow, while you train them horizontally.
Will the plants lift the net?: Yes! If it’s not secured it will be lifted. You need to fasten it in place.
How to fill the screen?: What you do is let the plants poke out the top a little bit, say 2-3” and then you careful pull it down from underneath, and move the end out along the bottom of your screen. Repeat this for all protruding growth tips. This process spreads the growth hormones more evenly in the plant and causes massive branching in most strains. Just keep training the plant like this unill your grid is like 70-80% full and then flower.