Fixing the size of the feature image on front page feature article layout

I am trying to set the size of the featured image that shows up using the "article gallery" format on the home page.

I looked at the settings and this is what I saw below

Choose your Featured Articles Thumbnail Options in homepage? * if 'attachment', thumbnail will be auto fetch from post attachment gallery first image

If you choose 'custom-field', enter your desired custom field key for thumbnails here
default: thumb

So I guess I should choose "custom-field"? If so, would thumb keep the images the same? What other options for custom field key do i have? I am not sure how this part of wordpress works.