In WordPress 2.5 or more, there is a built-in Gallery feature that enables the choice to include a picture gallery to some Publish or Page in your WordPress blog. (Ref: http://codex.wordpress.org/Gallery_Shortcode)

Use a size choice to specify the thumbnail size you need displayed. Valid values include "thumbnail", "medium", "large" and "full". The default is "thumbnail". How big the pictures for "thumbnail", "medium" and "large" could be set up in WordPress admin panel.

ie. [gallery size="medium"]

My Question: I am attempting to hack in the [gallery] shortcode to permit custom dimensions during the time of input -- not trying to get this done with the admin panel. Let me use something similar to, [gallery size="145x160"].

Instead of download a bloated wordpress plugin, I'd rather use what's already there and I am unsure where I have to use my file structure to create the alterations. I am acquainted with PHP but I am afraid I'll create a change after which after i update future versions of Wordpress, it'll overwrite what I have put in place.
Could someone assist me with this particular?

Thanks greatly!

to change size in pictures, you'll need use configurations ---> media

here you custom size.

Hopefully this assist you to!