Where do I enter a product's weight and size?

They go in the Shipping section of the product form, for physical products (Physical product turned on).

• Product weight, in Kg, gms or lbs.
• Dimensions (cm): Length, Width and Height.
• Package (optional): pick a saved box and its dimensions are filled in for you. Add or manage boxes under Settings → Store Settings → Shipping Package.
• Shipping cost, Country of origin and HS Code are also here.

For Variable products, set the weight and dimensions on each variant. Click Details on the variant's row.

Under Settings → Store Settings → Shipping Setup, Default Product Weight (kg) covers products with no weight (blank means 0.5 kg). Volumetric Weight Divisor sets how box size turns into chargeable weight.