There is technically no limit to the amount of text you can display for each image (although real-world constraints such as the size of the user's browser window will effectively provide a realistic upper limit).
If necessary, you could increase the captionHeight (in svBuilder-Pro's 'Customize -> Caption' section) to accommodate larger captions.
You could also choose to display your captions in an area other than the default position below the thumbnails using the captionPosition configuration option.
For possible values of captionPosition, please see its description in the Caption Options section of the Config Options page.

This can allow a SimpleViewer-Pro gallery to interact with the embedding page using the SimpleViewer-Pro API. You could run custom JavaScript code (for example to display text in a container outside the gallery on your web page) when svImageChange is fired (each time a new image is displayed). You could use the getImageIndex() method to determine what image is currently being displayed (to know when text to display in your container). Take a look at the online demo. You can view the source of the web page in your browser and copy or adapt the code to suit your own needs.