Overview
The carousel images size is standardized and there is no way to define them in the current version (London)
Example
As described in the following documentation, carousel user input use the image from the image library.
However, the "width" and "height" on the image record does not have any effect in the Virtual Agent carousel input. The system will keep the carousel at a responsive size so it fits in whatever space is available since the chat has to work on mobile, and many other locations.