I'm trying to present a bunch of MP4 videos in 480w x 360h resolution. Strangely, in the admin controls ZenPhoto lists them as varying & incorrect dimensions - 480x320 and even 320x240 (though all were encoded with the same codec/process/resolution).

Also, using the Flowplayer plugin - when I use the plugin options to increase the player size from the apparent default of 320x240, it does not increase after a certain (very small) point.

Any suggestions?

The current Zenphoto release is actually not able to read metadata from multimedia files (1.4.2 will), so the sizes you see is probably the size of the video thumbnail you are using.

The Flowplayer plugin has settings how to treat videos. Default is not to upscale larger than the video actually is.