The Dimensions template is the quickest way to fill length, width and height on every variant, for a shipping app that rates by size.
Before you start
Your store needs three variant metafield definitions of the type Dimension, one each for length, width and height. The app creates none, and matches nothing by name. If you do not have them, make them in your Shopify admin under Settings, Custom data, Variants.
Where
On Variants, the card Here to fill in dimensions? is on Choose fields and over the grid. Once set up, a Dimensions button sits on the Variants grid.
Say which fields hold what
Dimensions asks "Which of your fields hold length, width and height?", with a picker for each. Each picker lists your Dimension fields. Choose three different ones, then press Show 3 fields.
The grid
The Dimensions grid is the Variants grid with those three columns only. Type a number and pick a unit, paste a column from a spreadsheet, or drag a value down. There is no search, no filter and no CSV here: use the Variants grid for those.
Change fields returns to the pickers.
Review and apply
Review N changes opens the Variants' Review changes: the dimensions are variant changes like any other. Apply, and each value is read back. The result page leads back to Variants.
Then
Your rate app reads the fields. metangelo works out no shipping rate.