Magento does not have specific fields for weight, height, length or width but you can create "attributes" to store them against each SKU.
Using the default attribute names
If you name the attributes "weight", "height", "length" and "width", ReadyToShip will use them for consignment weights and item dimensions.
Using custom attribute names
If weights and dimensions have different attribute names, you can tell ReadyToShip which names to use.
Go to Settings ⇒ Stores ⇒ (your store) ⇒ Settings and instead of putting numbers in Force item weight to X (grams) and Default order dimensions, put the name of the Magento attribute to use.