I don't know if it's a good size but I send blocks of 20K until the size of the request or the connection drops.
Same, 20k blocks
User-Agent: Roku/DVP-2.9 (012.09E01529A)
GET /Ancient%20Aliens/Ancient%20Aliens_20110126_20002100.mp4 HTTP/1.1
curl -s -o output1 -D /dev/stdout http://blah
curl -r1-1 -s -o output2 -D /dev/stdout http://blah