None, images will fit to the slider automatically, but you can resize a slider if you want by adding height= and width= property to the,

What part of the code should I add that to?

<gallery height=HEIGHT width=WIDTH>
Some image.png
</gallery>

Change HEIGHT and WIDTH to any sort of measurement, like pixels or relative percentages to the screen

Thank you!

Actually for galleries it's widths and heights , in plural. Also this was used to have problems on the new platform, I hope they were fixed.