You will want to storyboard your movie and determine the scenes required . I suggest rendering out each scene as a seperate project and use a cheap or free video editor to put them together . A typical movie has clips of maybe 10 seconds with an occasional longer scene .

You can certainly do one in poser but it will take a while .

Render at 720 x 480 for full screen tv formats.

www.videohelp.com may answer some of your questions on video formats and editing .

Other packages may have more tools for animation , such as automatic  in/out curves but Poser will do it .

lets say a two minute short, at 15 frames per second,
thats 1,800 frames to render
if each frame takes 10 minutes to render (and thats pretty fast) you have 18,000 minutes
or 300 hours of render time alone .