"テニスボール大の団子" is suitable for the translation of "tennis-ball-sized dumpling".
"大" /dai/ means "just the same size or about the size of something", and, in this case, no one cares whether the dumpling is exactly same size as a tennis ball or not, while saying "tennis-ball-sized".
If you say "テニスボールと同じ大きさ", it means "just the same size as a tennis ball".
(Thanks to Kuribayashi-san for the comment)