We don't have any way to "lock" a fields position but I am not clear on what is happening for you, the only time things should "resize" is when you add additional fields to an existing row and they resize to fit all fields in the row, this behavior is set and can't be customized though. Is that what you are seeing, fields in the same row resizing?
When the labels are on the left, all first field of each row share the same label width. This is as designed.
What you can do is change the label position to be on top instead of on the left.
Another trick is describe in the post : http://forums.adobe.com/message/5754427#5754427 (see my last respond in that thread)