Sample code for Gareth that I'm using to size images used in a gallery.

Statamic Gallery

<!-- gallery.yaml -->
include:
 - .meta
type:
  prefix: number
fields:
  _template:
    display: Template
    type: hidden
    default: gallery
  status:
    display: status
    type: status
  page_content:
    display: Content
    type: redactor
  gallery_images:
    display: Gallery Images
    type:grid
    fields:
      title:
        display: Image Title
        type: text
      caption:
        display: Caption
        type: textarea
      img:
        display: Image
        type: file
        allowed: [png, gif, jpg, jpeg]
        destination: assets/img/gallery
  content:
    display: Content
    type: hidden

<!-- Template code (partial) below -->

{{ gallery_images }}
  		{{# <!-- portfolio item --> #}}
			<article class="portfolio-item four columns">
				{{# <!-- lightbox with image --> #}}
				<a href="{{ img src="{ img }" dim="960x800<" }}{{ image_url }}{{ /img }}" data-rel="prettyPhoto[folio]" class="img-link preload"
					title="{{ caption }}">
					{{ image src="{{ img }}" dim="420x210#" }}
					<img src="{{ image_url }}" alt="{{ title }}" class="scale-with-grid">
					{{ /image }}</a>
				{{# <!-- end lightbox with image --> #}}

				<header>
					<h1>{{ title }}</h1>
				</header>

				<div class="portfolio-excerpt">{{ caption }}</div>

			</article>
			{{# <!-- end .portfolio-item --> #}}
{{ /gallery_images}}