How large a generator should I get?

It depends how many appliances you plan to power. In an emergency I would be happy with 5 kilowatts which would power central heating (gas), lights, fridge, freezer and a kettle. Add up the wattage of all the things you feel are essential, and get a generator that is more powerful than that. 30A @ 220V limits you to a load of about 6.6Kw but if you want to power 6.6Kw of stuff I'd advise a 10 Kw generator of good quality.

30 amps of 220v, Forget about 110 crap when feeding a panel. Your feed/ your panel is basically 2 110 lines that each feed half the panel(basic term) You feed 110 into it and you'll have half your panel working, try to jump it and you'll have other issues. so just forget about it.

You need to decide what you want to power and for how long. Are we talking about keeping the refrigerator and a few lights on for a day or two vs. running the whole house for a couple of weeks in the event of a prolonged outage?

Generators are sized in watts or kilowatts (KW). A typical home may use between 10 and 20 KW at peak usage, but a lot less for the rest of the time, and less still if you're careful about minimizing usage. Your setup sounds like it would handle up to about 7,200 watts (7.2 KW) of power (240V x 30A = 7200W). Again, whether you need a generator that large depends on what you need and what you want to power during an outage.

30A x 240V = 7200W. You can get up to 30 amps maximum on each opposing 120v leg up to 7200 watts total. It will not deliver 60A on one leg. 240v loads draw half their watts from each leg, so a 240 volt 4500 watt water heater will draw about 2250 watts from each leg allowing 2700 watts, 1350 watts available on each 120v leg.

So something that can deliver a little more than 7200 would be good, like this http://amzn.to/2wfnFUm .

A little explanation about ratings. The generator has a peak rating, normally the generator is absolutely not capable of delivering any more than that. The generator can deliver the peak for only about 15 seconds or so, then it will trip. The trip curve on a circuit breaker in your panel is a more lenient than that, it will allow 1.5 to 2 times the rating for 1 minute before it will trip, and it could deliver 1.5x the rating for 15 minutes and still be within UL spec. The issue is about heat, generators can overheat, so they use a pretty tight trip time curve, household wiring is de-rated to begin with, so the time curve can be more generous. What you want to prevent is motors struggling to start, slow turning motors overheat easily.

So if you want the most trouble free power available and budget is not too big of a concern then it wouldn't be completely unreasonable to get a generator with a 10,000 watt running capacity.

If budget is an issue, and you only expect a few hours at a time, then you could get away with less, you will just be limited to the total power available from the generator, and the breaker on the generator will trip if you exceed capacity preventing most damage. But be careful, some smaller generators like this http://amzn.to/2wG0sMc don't have a 240v connection.

I used a 5500 watt peak generator for 13 years at my last house. We often had wind storms that took down our power lines and it took several days before restoration. I could deal with it, only running minimal general purpose circuits, turning off fridges/freezers, and one 30 amp 2 pole breaker at a time. I could do it, but not my wife. (Turn on well pump, then well off, water heater for two hours, heater off, pump on, take showers, then turn on fridges.) Now we live in a house with gas appliances, and use a 3500 watt generator, I can only run one 1500w appliance at a time, the only thing I really can't do is run the clothes washer.

yep, 6,000 watts is the max you can bring in.........and get an electrician to check out whatever you do, esp the part about "main breaker open" so you dont fry linesmen in the area.......what you need is a disconnect/transfer switch........