doesn't really matter. if you use lpd8806, (sounds like you want to), all the processor does is copy data from the serial port to the spi port. for the ws2801 data is buffered, but only up to 256 bytes. i guess it's conceivable that with a really large screen you might have to increase the buffer size, but, if you cranked that buffer out to 1k, that's 341 and a third leds. i'm not sure how the buffer size corresponds to the maximum number of leds supported, but the buffering is done because with the ws2801 the output has to be faster than the input because latching data to the outputs occurs any time there's a pause greater than 1ms, (i think).
so how many leds is it going to take to go around that screen, anyway?
"If I had known it was harmless, I would have killed it myself." - Phillip K. Dick, A Scanner Darkly