As long as you set screenMode="AUTO" (AUTO is the default value for screenMode) in JuiceboxBuilder-Pro's 'Customize -> General' section, then Juicebox will determine the best screen mode (SMALL vs LARGE) for the device being used to view the gallery.
Juicebox takes into account the device as well as the screen size when determining which screen mode to use.

Generally, with screenMode="AUTO", Juicebox will use Small Screen Mode on mobile devices (thumbnails and main images displayed on separate pages) and Large Screen Mode in desktop browsers (thumbnails and main images displayed together on the same page).

More information about screen modes can be found in the Gallery Tour.

If you have some unexpected results, then please let me know what you see and what you expect to see and what devices and browsers you use.
If there is a problem, then I might be able to help.
Otherwise, I might at least be able to confirm whether or not what you are seeing is expected behavior.
Thank you.