Word of warning, Premiere doesn't seem to like rendering any changes like this to captions. To make any changes take effect I'm having to save, quit and relaunch every time. Pretty fun.
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I think I've got a fix for this. If you hold cmd/control while dragging out the end of a caption it ripple shifts all the captions after it. So you can do this for the size space you need then bring the one caption back down to size and insert the new captions in the space. Voila!
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