I have it on Steam. It's about 30GB in total, and that's with every expansion and stuff pack included. I recommend getting the game on Steam since the Steam version is cheaper, it goes on sale more frequently, and it comes with codes for Origin / the EA Launcher in case you prefer to use that. I have to download the store content separately from the EA website though and it's hard to tell exactly how much data that takes up. Plus Custom Content and Mods can be a lot to download as well, and Vanilla Sims is sooo boring. Oh also the Steam version doesn't include Katy Perry Sweet Treats but that expansion is crap anyway.
So um, my point is, when you legally purchase the Sims 3 and all Expansions and all Stuff Packs and all the Store Content for $10,000 or whatever ungodly amount of money it costs (I am going to assume you intend to legally purchase the game. Piracy is a crime-- even if it's very easy and people rarely get into trouble for piracy, I am a law abiding citizen so I am not going to tell explain piracy, even if my refusal is somewhat silly because piracy can be literally one Google search away), make sure you have between 30 and 40 gigabytes of hard drive space to spare. That's probably enough, right?