Hey Paul...
What are the favourite sizes?
Many people are loving the full screen, that goes without saying why. Standard sizes like 320x240 seem to convert well for me. I would say, the bigger the better.
What are the favourite formats?
Wmv is first followed by mpg (in my experience).
What are the favourite length of the clips?
1 minute clips around 3mb.. or full clip of 10 minutes work best (again just my opinion). For webmasters making galleries, 10-20 second clips around 1mb worked best in my promotion.
Does any know or can recommend a good splitting and conversion program?
I use
BoilSoft Video Splitter.
Just my two cents, but hope it helps.