In the carousel using the FlickrRSS and NextGen Gallery modes, image sizes are set by Flickr thumb sizes and the NextGen Gallery plugin respectively.
From WP-Admin, click Gallery, then Options, then on the Thumbnails tab. Change the dimensions to what you want and click "Save Changes".
Now, you have to do this for each gallery:
Click Manage Gallery (on the left), click on the title of a gallery, check the checkbox at the top of the images (which will check all images), then from the dropdown that reads "no action", choose "Create New Thumbnails", and then click "Apply" (just to the right of the dropdown). This will generate new thumbnails at the chosen size.
There are options inside NextGen to add titles/captions to the images.