Have you already contacted ProPhoto for tech support? You get free support with that Theme:
https://www.prophotoblogs.com/contact/
Regarding Slideshow image sizes, this might help make some sense of things:
The ProPhoto Slideshow player size is calculated in a unique way so that your images are displayed as best as possible without any cropping. To figure out the best way to display your images, the pixel width and height of all gallery images will be checked. The player will be set to a width and height based upon these criteria, in order from most important to least:
1) If all gallery images are too wide for the area the Slideshow is to appear on your site, they will all be downsized to fit in the area the Slideshow is placed.
2) If all image widths fit in the space the Slideshow should appear, the widest image will be used to set the width/height of the player and all other images will be downsized to fit within this width/height.
3) If there happens to be more than one image with the same ‘max-width’, the the player will be sized to this width, and the height will be decided from the most common ‘aspect ratio’ found among the wide images. For example, if there are 3 images at 900×400 and 6 images at 900×350, the player will fit the majority at 900×350 and the other 3 images will downsize 50px to fit the height. This will cause these 3 images to appear narrower than the 900×350, even though they had originally been the same width of 900px.
It is advised that you use images of the same crop aspect ratio for best results, but if you must mix-and-match aspect ratios, you may find that you need to prepare photos taking this into consideration – ProPhoto Slideshows try to optimize the player size so that ‘most images display as large as possible.’
