Well, I can't say for 100%, but Italy has 1.9 Trln in debt - this is greater than Portugal, Greece, Spain and Ireland all together. If we assume that Germany, France and other "old" Europe will not need rescue, then rest is Eastern Europe.
Applying some simple calculation, I suppose, that somewhere around 3-5 Trln. will be enough
Still, the core of solution - is ability to buyback problem bonds by ECB. If EU Government will not provide this ability, then, probably EUR will not hold, at least as we know it now.
Most probable scenario in this case is contracting of EU to former "old" area - ECU, and excluding other countries. Italy anyway will be saved. Currently such kind of views more and more are spreading across the Europe.
In fact they have to start the same procedure as Fed by its QE programs - printing money.