Big Maps aren't my thing. The people I play with like 5x5, 10x10 and MAYBE 20x20. Best is Eco Spam which is why I don't like it.
I've RARELY even SEEN a T3 mex. Getting your Mex's to T2 is very important and THATS when you build storage. A T2 Mex surrounded by storage gives you +9 instead of +6.
Yes, mass is the most important thing in the game. If you are planning on saving up for something...like a Tac Missile Launcher for a snipe or a Com upgrade, saving is great. Otherwise try to stay at about -5 at the start, when you have many T2 mexs, I go to about -20. But you get a feel for it.
If you play with me, be very careful with T2 Pgens. They are PRIME targets and cost a LOT.
Turtling can and will get you killed. Every game. Don't use defencive stuctures as units are much better.If you don't use T1, and I do, you lose. Hell, my starting build queue for my first Fac is:
2 Engies, 2 LABS, 1 Scout, 2 Engies, 2 Manti, 2 Engies, 2 Manti, 2 Engies, 200 Manti.
Try to make sure you are always doing something. NEVER Wait for something to finish. If you have an army, don't let it sit somewhere. Attack. Harass. Move to a better spot. The only time I can think where it should be still is a standoff.
SCOUT! EXTREMELY IMPORTANT! Get a radar up fast. They're cheap as hell. Throw em up all over the place.
Use your commander offencivly. He is a BEAST and as long as your careful can win you the game. OC in groups. Its AoE. Just ensure you're always progressing toward a goal with where you are moving him and make sure you have the area up-ahead scouted out.
Just a sidenote, a Com with the T2 engeneering upgrade also reclaims a LOT faster. You can reclaim a factory faster then you can shoot it down.
Sorry, it might sound random, typing with Alt-Tabing at work.