I have one that's 3m by 2m, but that's for my live action work at school/college/uni, and even that is way too big for what I need, so I got a pop-out one too.

For brickfilming though, I would honestly just suggest A3 card. I can't imagine you're going to need anything bigger than that for brickfilming. Anything else would be a waste of money.

"Nothing goes down 'less I'm involved. No nuggets. No onion rings. No nothin'. A cheeseburger gets sold in the park, I want in! You got fat while we starved on the streets...now it's my turn!" -Harley Morenstein

Normally, yes. Unless you're adept at keying (which, after 7 years of learning, even I'm not) then a keyed shot has a 50/50 chance of actually coming out in post (the lighting has to be perfect throughout).

I would suggest printing out backgrounds. I normally do that myself, but on A4 rather than A3, as it saves a lot of ink and most of my sets aren't that big. You can see what I mean in these videos:
https://www.youtube.com/watch?v=gai7NOEtcfY
https://www.youtube.com/watch?v=J1txbwsB8pg
https://www.youtube.com/watch?v=-6Wl7L5NFMs
If you want any of those backgrounds (though none of them are that great in truth) I can make a Dropbox folder of them for you, though I'd recommend checking out this site. I get most of mine from there.

I've only really succeeded with keyed backgrounds twice - in this and this. I'd really suggest going for printed backgrounds until you're a bit more experience, and even then use it sparingly.

NickDurron, Smeagol and Metru Nui Legacy are a few of the people who've managed to get it right and I'd suggest checking out their work (particularly MNL).

"Nothing goes down 'less I'm involved. No nuggets. No onion rings. No nothin'. A cheeseburger gets sold in the park, I want in! You got fat while we starved on the streets...now it's my turn!" -Harley Morenstein

I suggest a paint color card. Go to your local hardware store or Walmart to get one.

But, get the solid color ones, not the multi-color ones. They are pure colors, which no inconsistencies, they are fairly thick, and get the job done well. Plus, they don't cost anything (but I don't recommend taking large amounts or them, or taking one without buying something from the store, because the store paid for them and taking them without buying something wouldn't be right. ). If you need a larger screen, look no farther than poster board. It's really cheap. You can go to the craft store or Walmart for that too. But remember, if you put sticky-tac or clay on the posterboard or the color card, it will leave a mark.

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"Whatever you do, do all to the glory of God." - 1 Corinthians 10:31b

I use a bright green piece of posterboard from Walmart. It was, like, a dollar. It's probably not more than 1' by 1.5'. I suppose a larger one would be advantageous as you could reduce spill by putting it further back.

Yeah, I was thinking of doing this exact thing. I have 2 large green screens for photography and those can be a pain, what with wrinkles, etc, but posterboard would work wonderfull!

Mine's like 7 by 12 feet, and vinyl.

I have a big ol one. a small one is fine, UNLESS, you wanted to do wide shots, or ones with planes and stuff and you dont want the vehicle reflecting the green, so you want it further away, but, I think something like a 1m x 1m will be plenty.

And dont get any that is shiny.