Hi Gil,
You have to get info for large LED driven billboards. I checked with few of manufacturers last year, the price for a 3x4 meter was above US$200,000.00.

If you plan to develope such display, then you need to use ultra-bright LEDs for outdoor use. You will need Red Greed and Blue LEDs and convert a regular Video signals to the digital format and drivers for the LEDs.

Due to response time limitations you need to devide your full screen size into several portions and devide the scanning method to all portions individually.

Rauf