You increase the thumbnails by increasing the size of the thumbnail bar, itself, rather than a slider.
Hover your mouse on the horizontal boundaries between the thumbnails and the main Loup area and one will turn it into a double-arrow. Click-drag this and the thumbnails will resize.
Thanks muchos. Knew it had to be something simple.
Hover the mouse over the top edge of the filmstrip until the mouse cursor changes and then
- Double-Click the mouse, will toggle the height of the filmstrip between the last two settings!