how big has to be an all-in raise to reopen the betting?

[jjacky]

at least 50% or 100% of the last raise?

[SheridanCat]

As with many poker issues, it depends.
Here's a thread where Randy Refeld discusses this very thing. You'll see that the answer varies depending on locality, quality of the floor and whether you're playing limit or no-limit.

I found this, and a couple other threads by doing a search using the following terms:

+all +in +reopen

I hope that helps.

Regards,

T