There is no such option unfortunately. Due to a 2 reasons I've mentioned above:

1) The albums module in UNA does not provide a custom sized thumbnails for photos. Instead it provides a fixed size/aspect ratio thumbnails to be used in galleries and previews. The Photos Carousel module does not create its own photos, it just uses what the source module offers.

2) All items in the Photos Carousel must be of the same size for the transitions and animations to work correctly.