Hi there,
This feature has not yet been added. However, the original concern (more images increasing overall file size) is somewhat mitigated. If you have two instances of the same image, and one is scaled down (thumbnail) and the other is full size, then SmartBuilder is smart enough to only include the larger file with the package - it doesn't bundle 2 copies.
Overall, SmartBuilder tends to be WYSIWYG (what you see is what you get), so rather than setting the size in the actions, previewing, adjusting size and position in the actions, and then previewing again, and making more adjustments... and then you still have to reset those size/position changes in the actions when the zoom is closed. It tends to make more sense to just have 2 image objects on the page that you can see simultaneously, and a simple action to show / hide the large image. When you're happy with the larger image, you can simply hide it in the Cast to create a clean workspace.
In fact, here's a super easy way to create a toggle visibility of an object in the actions (see attached image). The "not" block is found under the "Value" action category.
If there's a different need for setting the size of an image in pixels that we're not seeing, let us know!