How do I make captions bigger?

Increasing font size does nothing on screen and 'transform' doesn't seem to apply to captions either. 

Closed Captioning is a standard size and has pretty strict limitations. How big are you trying to make them? Captions have their own editor so "Transform" has nothing to do with CC.

I think the only way to have full control is transforming the captions to titles using apps like X-title extractor or FCP SRT importer. I do this most of the times cause in The Netherlands we use white subtitles with a black outline.

Captions are embedded code in the video file. Subtitles are simply text burned in on the screen. So not sure which you're trying to do.