UPDATE: It would seem that someone helpful reads this blog! A big thanks to commentor Troy, that implemented a fix for this issue. Look for a Match Length button in future releases.

When importing a movie into the Blender Compositor (2.57), there is no way to know how long a movie is (in frames).

Go to a texture panel (theres an easy to access one in world attributes), add the same movie there. Then in the image Load window, click the button “Match Movie Length”. Use the number from there in the compositor node for the movie length in frames.

Bonus tip:
Alternatively you could load a bunch of movies into different texture slots, in the texture panel and perform the “match frame length” for each one. Then goto the movie input node, and select the appropriate movie from the drop down list.

Match Frame button for clip length

This would save you re-loading the movies in nodes from the open file dialog. Currently there are some issues with using the compositor’s output in the VSE (limited access). But at least you can render footage out from the compositor and add it as a strip.