How to change the Width of Tiled Gallery Area

I'm using jetpack (or maybe slimjetpack) and am wondering how to change the width of the tiled image area.

I have about 600 pixels but the tiled images (regardless of how many columns it is) only takes up about 450-500 pixels.

Thanks!

Susanna
P.S. If I change the settings for this while using slimjacket and switch to jetpack when my site is no longer password protected will I have to change the settings in regular jetpack.

(I'd rather use regular jetpack just in case the folks who manage slimjetpack decide not to update it one day). I don't want to create extra work for myself.