In my experience $5,000 would be the *absolute* minimum, but I'd expect the seller to counter asking for more. $10,000 - $25,000 would be more typical. I've seen as high as $50,000.
You want to convey seriousness to the seller (I'd recommend $10k as a reasonable minimum figure for that), but also recognize that you it's *possible* something could happen to cause you to unintentionally default. I've seen buyers have motorcycle accidents, lose their jobs, get sick, have a death in the immediate family, etc. Some just get cold feet and change their minds!
However unlikely, if something like that happens you could also face losing the hand money (in addition to the original calamity). So that's an argument against making it *too* high. Also, it's tied up in escrow for however long the deal takes to close, and isn't earning interest in the meantime. It should be high enough to show you're serious about the house, but not so high it would bankrupt you if you lost it.
I've had people who wanted to put up close to the entire purchase price (for low-priced investment homes, not $700k, of course), in one case just because they didn't want the bother of writing two checks, believe it or not, but I advised against it as an unnecessary risk. Good luck!