A newbie with a cool idea

[ggstarfoxxie]

Hello everyone. I've played alot of custom stories & have been inspired to make my very own. It's going to be based in Starcraft universe about the terran's (humans) evil Confederacy & its experiments on the living.

However, I have absolutely no experience with map making, modeling, or scripting. I have a bunch of questions though so bear with me.

1. I have plans to make new monsters, but I don't have Maya. Is there a free 3d modeling software that's compatible with the editors used for Amnesia?
2. How can I tell how big the monster will be in the game? Is there a standard scaling used across all 3d programs? I don't want to have an epic monster be like 2 foot tall.
3. How big can a map be without seeing lag because of all the stuff in it?

I realize this maybe too much to learn at once, but just want to be one of the cool kids that make epic stories of epic proportions.