Hi there,
The best way to keep the images size same is to use images of same dimensions, e.g., 800×400 pixels.
As for adding images in bulk, the best way to create a WordPress gallery (see: http://prntscr.com/9qdtb7). It will give you following result: http://prntscr.com/9qdu8c. The lightbox selector to target the gallery images, would be:
[lightbox selector=".gallery-icon a"]
So the complete example code will be:
[Gallery columns="4" link="file" size="full" ids="7377,7552,7551,7558"]
[lightbox selector=".gallery-icon a"]
To find out more about WordPress gallery shortcode, review this article: https://codex.wordpress.org/The_WordPress_Gallery
Thank you!