Example: I record a sequence with a green screen and a person talking. The recording is in 1920x1080 pixels. After the keying and editing the movie I would only output the person talking, let's say 700x1080.I had no problem to do it with FLV since there is a "Resize Video" option on the Media encoder. I haven't found that on other export settings, e.g. H264, mp4. There it let's me only do the given aspect ratio of 1.777777... or 1.33333...If I try to force it into another format by using the width/height entry, it comes an error "invalid frame size".
What I would like is a video that has a different aspect ratio than 16:9 or 4:3, a cropped version in the mp4 format but not using FLV.
Create a new sequence using the frame size you want. Copy the footage you have to the new sequence and scale it accordingly using the scale tool (under Motion in the video effects panel). Export the new sequence.
Use the Crop feature of the Export Settings (Source tab, top left) and then change the dimensions of the export. (But do keep them evenly divisible by 8 for best results.)