I haven't used Sanity before, so it took me a while to figure out, how to get photos dimensions using GROQ queries.

Here's how I've done it:

GROQ query

export const query = groq`
  *[_type == "photo"] {
    image{
      "dimensions": asset->metadata.dimensions
    }
  }
`

Response

{
  image: {
    dimensions: {
      _type: 'sanity.imageDimensions',
      aspectRatio: 0.6825192802056556,
      height: 778,
      width: 531
    }
  }
}

Fetch example

export async function getStaticProps() {
  const dimensions = await sanityClient.fetch(query)

  return {
    props: { dimensions }
  }
}

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• Next.js and Sanity Starter
• Sanity: Image metadata
• Sanity: GROQ query for image palette