In your examples they are better without the 'a'. The second example made me chuckle. When I go to a restaurant to buy breakfast and ask for pancakes they are always, and only, round and flat.
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Russkiy Bear1) If I always use "of" in such cases, will it always be correct?No. 'Of' is unnatural
Russkiy BearWhat size (a) sofa did you buy?No article.