If the writer was pointing to the design of the suitcase, he or she might say "this is a new design". If not, the style of writing is a little lazy and a better thing to say would be "its design is new". "It's a good size" is shorthand for "it's a good size of suitcase" or "the size of the suitcase is good". Using expressions like "it's of a good design" are quite formal and are not necessary, in my opinion, because simpler expressions are clearer and easier to understand. .