"I see" is right.
1. Use USCIS, for my case, I use
Department of Homeland Security
U.S. Citizenship and Immigration Services
2. The length is usually around 1.5-2 pages. If you think that is necessary for longer, you can.
3. ETA-750 B is necessary for EB2. ETA-750A depends on USCIS officer's preference. (For my case, I was told to submit ETA 750 Part A & B since I did not submit ETA 750 at all.) I think atoney may help you avoiding any mistake like those happened to me (I am self petitioner)
Good luck.