How to query a single image size with upload plugin?

You can query through upload_file model using id for getting the required data.

const Data = await strapi.query('file', 'upload').findOne({id : data.image});
 
console.log(Data.size)

Where Data have response like

{
  id: 799,
  name: 'download.png',
  alternativeText: '',
  caption: '',
  width: 177,
  height: 99,
  formats: null,
  hash: 'download_4b63425cad',
  ext: '.png',
  mime: 'image/png',
  size: 2.18,
  url: 'https://ww.abc.com/mediafiles/blogs/download_4b63425cad.png',
  previewUrl: null,
  provider: 'local-provider',
  provider_metadata: null,