The default resize dimensions for medium size images of 1024 x 768 usually give a good compromise between image quality and image size (which has a bearing on speed of loading).

There's certainly no point in increasing the resize dimensions to be larger than the dimensions of the source images. If the source images are smaller (in resolution) than the resize dimensions, then JuiceboxBuilder will just copy the source images across to the output gallery directory (it will not upscale them).

However, you might like to increase the resize dimensions (maybe to 1080 or 1152 wide), especially if your site's target audience is likely to be viewing your images on high resolution or high pixel density monitors.
Juicebox itself will dynamically downsize the images (if necessary and as long as imageScaleMode is set to SCALE_DOWN or SCALE) when the gallery is being displayed/viewed to fit within the gallery's image area so users with smaller monitors will not notice any difference (other than the fact that the images might take a little longer to load) but users with larger monitors will benefit from the higher resolution images.

With internet connections becoming faster and faster these days, the slightly longer loading time for larger images might not be an issue for most users but it is something to keep in mind as not everyone has a fast connection.

Having said that, Juicebox-Pro preloads images (see imagePreloading in the Main Image section of the Config Options page) so any image loading delay for larger images will be minimal and, if noticeable at all, may only be apparent on the first image.