Can I broadcast live video for 1000 users?

You should have no problem serving a stream to 1000 users on a single server (a dual xeon/2bg ram box ought to do it).

Your bandwidth requirements will depend on the bitrate of the video stream. Multiply the stream bitrate by the number of users (plus 1 for the publisher), and you'll get your bandwidth figure.

For example, if the video is published at 300kbps, you'll use about 290mbps for your 1000 users.

For the video size, I wouldn't go much larger than 640x480. The flashplayer doesn't use the video card to render video, and redrawing the screen with a large video chews through lots of processor cycles very quickly.