larger pixel dimension galleries

There are no real problems associated with having larger images in your galleries (other than the larger file sizes which result in the need for more server storage space and longer download times).

Assuming your galleries are Juicebox-Pro galleries:

(1) Images will always be dynamically scaled down to be displayed in their entirety, no matter what the size or shape of the user's viewport is (as long as imageScaleMode is set to SCALE_DOWN (the default value) or SCALE).

(2) Images are preloaded (according to the imagePreloading configuration option value) so while one image is being viewed, the next few images are being preloaded in the background so any delay associated with larger images will be minimal and may only be apparent on the first image. (The default value for imagePreloading is PAGE (whereby all images on the current thumbnail page are preloaded) but you can change it to NEXT (the next image only) or NONE, if you like.)

(3) You could have a Multi-Size Image gallery whereby your gallery can have up to 3 different image sizes and Juicebox-Pro will display the size most appropriate to the user's device and screen size.
GO to 'Site -> Customize Viewers -> Juicebox-Pro (Edit)', then scroll down to 'Pro Viewer Options' and expand the 'Multi-size Images (Pro)' section for the options.

Short descriptions of the configuration options in bold above can be found in the Main Image Options section of the Juicebox-Pro Config Options page.

I hope this helps.

I've just uploaded a fresh Showakse v1.7.6 site and checked the multi-image size functionality. It seems to be working OK for myself.

Here's what I did:
(1) Site -> Customize Viewers -> Juicebox-Pro (Edit)
(2) Selected 'Large images', 'Medium image resize' and 'Small images' checkboxes.
(3) Created a new Juicebox-Pro gallery page (on 'Pages' panel).
(4) Clicked 'Upload' tab.
(5) Uploaded a single image (dimensions 4416 x 3312) via upload module (via HTML 5 method).
(6) Published site (clicked 'Publish').
(All image sizes and all other checkbox were left at their default values.)

Now, when I check inside the Juicebox-Pro gallery's folder (in the root Showkase directory), there's a folder named 'images'.
Directly inside this folder is the medium image (dimensions 1024 x 768).
Also, inside the folder is a subfolder named 'large' which contains the large image (dimensions 1280 x 960).
Finally, inside the folder is a subfolder named 'small' which contains the small image (dimensions 800 x 600.).

It is worth noting that Juicebox-Pro uses the medium images for a gallery being displayed on a desktop monitor (no matter what the size of the monitor is). Large images are reserved for when an image is opened in a new tab (via the Open Image button) or downloaded (via the Download Button). Small images are used on mobile devices, though (unless a retina/high pixel density screen is detected, in which case, medium images will be used).

Give it another go and hopefully it'll work OK for you as it does for me.

The logic that Juicebox uses to decide which image size to use is hardcoded into the 'juicebox.js' JavaScript file which is obfuscated and cannot be modified so you'll not be able to do anything there.

The best course of action would likely be to increase the resolution of your medium images and perhaps reduce the quality percentage a little ('Site -> Customize Viewers -> Juicebox-Pro (Edit) -> Pro Viewer Options -> Multi-size Images (Pro)') in order to keep the file size at a reasonable level.
If you don't want your images to be downloaded, then a slight decrease in quality might be acceptable. The likelihood is that a slight quality percentage reduction will result in a smaller file size but the visual quality may be hard to notice(at least to a casual viewer). A little bit of trial and error might be required to find a balance but I hope that you can find a suitable compromise between resolution and file size.