Some codecs require a multiple of 4 for the render size. If iClone uses Microsoft MediaWriter then this is indeed the case:

MP4 videos created using the MediaWriter libraries must have a width which is a multiple of 4 and a height which is an even number. If the width is not a multiple of 4 the video will be automatically trimmed by removing pixels from the left and right to make the width a multiple of 4. If the height is not an even number the bottom row of pixels will be discarded.

Since 690/4=172.5, the size would indeed be trimmed to 688. So this is a limitation outside of RL's control. 

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