At this time, the ability to add size and color to a product detail for the inventory is unavailable. QuickBooks Online (QBO) doesn't have a Unit of Measure feature that helps to manage inventory items with different units.
If you have already an inventory item, you can edit it and add the size and color on the description section. Let me guide you how:
Go to the Sales from the left menu.
Select Products and Services.
Locate the Inventory you create.
Under Action, click Edit.
Enter the Size and Color under the description section.