It varies - save files are a lot bigger than the game. The game files take about 50 megabytes. Saves start out around a megabyte, but they can work up to hundreds, if not thousands. For a well developed world, 10 gigabytes isn't out of the picture.
If you're asking about memory, lots. I have 8GB of RAM and it runs fine, but I've tried it with 2 and had a fair bit of lag. At least 4 gigs of RAM for optimal performance. However, the game isn't intense enough that a little bit of lag ruins it.
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Between 19.45 to 25.46 mb
A little bit above 1 GB
Minecraft's recommended amount of specs is to have at least 1 GB of space, however the absolute bare minimum is 500MB. With 3 - 4GB's, you should be good to play on some higher settings!
Fix your Java, for example if you have a 32bit pc, use 32java, if you have a 64bit pc, use java 64 bit, or clean out your cache and memory, minecraft needs to have good performace and good backup space (~1 GB)
You only need one, but you would need lots of DDR3 (RAM), maybe 2+ GB.