usually 512mb-1gb for the free ones. And flash drives will probably never overtake discs. Flash drives are more efficient to use but discs are faster to press and also continuously getting cheaper.
Also:
Well there's still a long way for flash drives to overtake discs but you can expect them to take over traditional hard drives in near future.
This is still a long way off, actually (in computer ages at least), solid state drives are still ridiculously expensive (although way more stable and faster) than traditional hard drives. A SSD costs about 3.50 USD per gigabyte of data and are up to 256gb in size (which would be almost $900), while an HDD currently costs about 40 cents per gigabyte and can reach into terabytes of data. They also wear out after so many writes to them. Basically, at this point, if anyone recommends a SSD, they're recommending a 32gb one for your Operating System only.