I'd highly recommend you read Chris White's The Cruising Multihull
Generally, you need to reef multihulls for the gust strength, not the average wind strength. Unlike a monohull, the multihulls can't shed the extra wind in the gusts by heeling—at least not to any significant degree.
It is a bit hard to say what is the best way to sail a catamaran, without knowing more about what catamaran it is or what the conditions are like.
The biggest danger for most multihulls, provided you don't have them overpowered with too much sail up
, is pitchpoling them.
As for what kind/size/period of wave is needed to capsize a catamaran—it really depends on the design of the catamaran. One with a narrow beam, a solid bridgedeck that comes all the way forward, stub keels, lots of freeboard, and a low bridgedeck clearance is going to be in far more trouble than one with a wider beam, a bridgedeck that doesn't come as far forward, a higher bridgedeck clearance, centerboards, and less freeboard.
Another characteristic that makes a difference is whether the catamaran has keels, centerboards or daggerboards. In really heavy seas, cats can often trip on a keel and flip sideways. A lot of cat sailors I know think it is a really good idea, unless you need to sail to windward because of a leeshore, to raise the boards and let the boat slip more, rather than risk tripping her up.
One of the best pieces of safety gear for any small boat IMHO is a Jordan Series Drogue
. It will slow the boat down in really heavy weather and help prevent it from pitchpoling or capsizing.