Usually yes. Boxes from the same supplier with the same structure and material, differing only in length, width, and height, can typically be combined into one Declaration.
The condition is: the material composition and sources of evidence must be consistent, and the "packaging identification" section of the document must state the scope covered.
Example of how to state it: "Corrugated outer carton, five-ply, recycled kraft, supplier ABC Paper Co., covering article numbers CB-0800 / CB-1000 / CB-1200 (dimensional range 800–1200 × 300 × 800 mm)."
Important reminder: where to draw grouping boundaries is a judgment call, and the regulation does not provide a clean method. Grouping too broadly will be questioned by the buyer, grouping too narrowly will exhaust you.