The Jobs > Tools > Import Items feature has been updated to handle importing multiple sets of dimensions for an Item.
This is achieved by duplicating rows detailing Items in a spreadsheet and including unique length, width and height values for each row.
In the screenshot below you will notice that the loaded spreadsheet's rows with ID 1 & 4 have 3 & 2 rows respectively (indicated by purple arrows) with unique sets of dimensions for each. To import the sets of dimensions for their respective Item you will need to set the Split ID column mapping to the repeating ID column in the spreadsheet (indicated by the pink arrow) along with the corresponding length, width and height columns. Finally, you will need to specify if the dimensions are in imperial (inches) or metric (cm) units.