Juicebox does not actually support responsive thumbnail dimensions. There is no configuration available which allows thumbnails to change size depending on the available space. The number of thumbnails displayed on-screen at any one time is responsive, though (see Note #2 below).
In all Juicebox galleries, thumbnails are always displayed at the designated thumbWidth and thumbHeight dimensions.
Space is reserved for the thumbnails in the gallery and the remaining space is used for the main image.
In the Top-Left Alignment demo, the gallery has fixed dimensions (1000px x 1000px) and is therefore not responsive at all. The gallery is always displayed at 1000px x 1000px, regardless of the browser window size. If the browser window is made smaller, the gallery is simply cropped.
The only way I can make anything in the Top-Left Alignment demo smaller is to change the browser's zoom setting (which affects all content on the page, i.e. the main image and thumbnails). Just changing the browser window size has no effect on the dimensions of the main image or thumbnails.
Bearing in mind that there is no way to make thumbnail dimensions responsive within a Juicebox gallery, here are some notes on gallery responsiveness and thumbnails which might help.
Note #1: Gallery Responsiveness
There are essentially two ways to make a Juicebox gallery responsive (with its dimensions dynamically changing with the size of the user's browser window).
A Juicebox gallery will be responsive (and will dynamically scale with the size of the user's browser window) if the gallery's own dimensions and the dimensions of all parent containers are expressed as percentages. If there is a fixed value anywhere up the chain, then the gallery's size will become fixed (e.g. 100% x 100% x 800px = 800px).
Please note that when using percentage heights, you may need to implement the suggestion noted here.
An example of this can be found in the resizable galleries support section here.
Take a look at the source of this sample gallery in your browser to see how this might be achieved.
Note #2: Number Of Thumbnails Displayed
When thumbsPosition is set to either LEFT or RIGHT, the number of thumbnail columns is fixed at the maxThumbColumns value whereas the number of thumbnail rows is variable (up to the maxThumbRows value) depending on the dimensions of the thumbnails and the space available in the user's browser.
Take a look at this demo gallery with 3 columns of thumbnails.
When thumbsPosition is set to either TOP or BOTTOM, the number of thumbnail rows is fixed at the maxThumbRows value whereas the number of thumbnail columns is variable (up to the maxThumbColumns value) depending on the dimensions of the thumbnails and the space available in the user's browser.
Take a look at this demo gallery with 3 rows of thumbnails.