1) Choose Product, Weights & Dimensions from the main menu within the admin console.
2) Use the filter at the top of the page to find the product (or range of products) for which you want to enter shipping weights and/or dimensions.
3) Enter your data directly on the page. Remember the following points:
Your unit of measure (for dimensions) is controlled by a "Setting" called Localization.DimensionUnits
Your unit of measure (for weights) is controlled by a "Setting" called Localization.WeightUnits
Dimensions will not be passed to your carrier as part of Real-Time Shipping unless you check the box for 'ship separately'.
You can check the 'ship separately' box either with a left-click of the mouse, or by hitting the space bar.
4) To manage many products at the same time, you can import a spreadsheet containing your data. It is a good starting point to "Export All" (which exports all products and all variants along with their current weights and dimensions). If this is onerous, then the format is shown below. Please use .csv as the file format for best results. Weights and dimensions (and the flag for 'ship separately') are all stored at the variant level. The import will find the variant that you identify in your import file and add or update your new data. Import will not create or modify products or variants as part of its process, so please don't use this as a shortcut to add new products/variants. The exported file lists ProductName and VariantName only for your convenience -- these two fields are completely ignored by the import process.
5) The Bulk Weights & Dimensions page shows you one manageable page of variants at a time. Please be sure to use the SAVE button before moving to another page.