Current international limits are 50 W/m^2. The sun at noon is 1000 W/m^2, so by that standard the rectenna is going to be very large indeed.
50 W/m^2 is absurd! One of the biggest problems with solar power is how much space it takes. Restricting yourself to 50 W/m^2 means that all things equal (which they wouldn't be, but still) you would be doing 20 times worse than normal solar power. For there to be any point in solar power satellites the flux in the beam must be much higher than the Sun's flux.
Are you sure 50 W/m^2 isn't just for some pathfinder experiments? It seems silly to have the same limits for radiation inside a power beam as everywhere else. It's a bit like having the same air quality standard inside a fireplace as in a city.