If the caption area is positioned on the overlay (captionPosition="OVERLAY" or captionPosition="OVERLAY_IMAGE"), then the caption area will dynamically grow or shrink to accommodate the caption text up to a maximum height determined by maxCaptionHeight (default value 120px).
If the caption area is positioned anywhere else (captionPosition="BELOW_IMAGE", captionPosition="BOTTOM" or captionPosition="BELOW_THUMBS"), then the caption area will always be at a fixed height determined by maxCaptionHeight .
If you don't mind your caption text overlapping the images, then set captionPosition="OVERLAY" or captionPosition="OVERLAY_IMAGE" and increase the value for maxCaptionHeight.
(The overlay can be toggled on and off as long as showImageOverlay="AUTO").
For reference, short descriptions of all configuration options in bold above can be found on the Config Options support page.
If you want to actually change the font size for the caption text, then please see this forum post:
Changing font size of image titles and captions (and gallery title)
I hope this helps.