MP4 saving

[surfer]

Whenever i load mp4's in Videostudio 5 or 6 and want to save the output as: create videofile same as first videoclip the message appears: the frame size (720 x 404) of the video file does not match the settings and therefore cannot be trimmed. This message appears on all mp4's and frame sizes when i want to make the output.
The mp4's are all H.264 High Profile Video at 23.976 frames /sec 24 bits MPEG-4 file extension mp4.

Then i want to custom the output to divx (6.9.2) but that allways failes. Xvid is succeeding though but there are a few things i would like to know why if possible:

Why is outputting the same mp4 not possible is it the codec of the frame size?
Which order to use when loading divx / mp4 in the output is it: upper field, lower field or frame size?
What is the best output format when outputting mp4 when the target is a mediaplayer (keep best HD quality off course)

I always have mp4 from the videocamera but also from downloading etc. So i would like to solve the output problem...