1. That is dependant entirely on flow rate.
2. I'd say a minumum of 200gph or higher, but really it depends on the sump and what equipment your running out of it.
3. Any design wich is used by people who have any common sense at all will not break siphon in the event of a power outage.... about 99.5% (totaly guessing on that though) of the time. With siphon - powered overflows it eventualy will happen. Keep murphys law in mind here.
The safest route is to use a drilled tank... this way the only way for it to overflow would be if the drain line get's clogged, a threat wich can be darn near eleminated by having multiple holes drilled and running multiple drain lines and/or an a emergency drain(s) wich would be at a higher level than the normal drain(s).