All of the articles I’ve seen talk about the possible (yet inconclusive) harm that radiation from your microwave (or other electronic device), but I can’t seem to find anything on higher wattage.
I know that > 600 watts would be like putting yourself in the microwave, which I would assume isn’t great for long term survival.
How about a narrow microwave beam of say, 40 watts? Would I be frying myself slowly, or okay?
You may want to look at microwaves and other forms of non-ionizing radiation here.
Well, supposedly the mesh present on all microwave doors is sized to prevent microwave leakeage. That is, the door acts as an antenna to ground out (capture) all of the microvave energy. I don’t know the specific frequency of microwave oven microwaves, but let’s say it’s 9GHz, giving us a 3cm bandwidth. Thus, any mesh with holes smaller than 3cm would absorb the microwave energy, thus sparing you from being heated up.
Now, how much is dangerous? Well, a microwave over uses full wave power, I imagine. On the contrary, the radars I used to work on in the army were 200,000 watts, and could barely cook a hot dog at the feed horn (where all the energy is concentrated); we tried it. On the other hand, this is 200,000 watts pulsed; the true RMS (how much real power) is a whole lot lower (got to turn off the power to wait for an echo to come back).
So, I’d be worried about 600 watt microwaves with the door open, but I wouldn’t worry about a 200KW radar. I imagine, having written this, that I don’t really have anything useful to say. 
For the record, the frequency used by microwave ovens is 2.45 GHz, smack in the middle of the ISM (industrial, scientific, and medical) low-power communications band. The ISM band is where 2.4 GHz cordless phones, wireless cameras, etc. reside (the up and coming Bluetooth protocol is also there).
Since RF at that frequency is non-ionizing, 40 Watts should just cause tissue heating. Depending on the size of the area it’s dispersed over, it may or may not be uncomfortable or dangerous. A 40 watt 2.4GHz beam that was spread over a person’s entire body might just warm them up a little, but 40 watts focused onto a small point could really heat up some tissue …
Also, try to avoid looking directly into microwave horns or other microwave transmitting antennas, as they tend to tightly focus the beam, and if you’re looking into one you’ll naturally look right into the center of the beam.
Arjuna34
Wasn’t it by observing radar setups that the idea for the microwave oven was discovered? As I’ve heard the story, a technician noticed that a candy bar left near the unit melted, which inspired him to try (and succeed) popping popcorn. Are chocolate and popcorn just a lot easier to heat than nitratified pseudo-meat tubular products?
Oh goody, I get to bust the moderator for perpetuating an urban legend.
The microwave effects were discovered when scientists were having trouble with a new radar that did not work in foggy conditions. And that was the whole POINT of radar, to be able to see despite bad visibility. But this was a new radar on a new frequency.
The problem was eventually attributed to water molecules in the air absorbing the microwave energy. They did some experiments blasting these frequencies at water vapors and discovered the temperature of the vapor increased. A few experiments later, and the microwave oven was invented.
Now I freely admit this was just something I heard. But I heard this from MIT scientists when I visited the Rad Lab where the effect was discovered.