  We study the structure of anticodes in the sum-rank metric for arbitrary
fields and matrix blocks of arbitrary sizes. Our main result is a complete
classification of optimal linear anticodes. We also compare the cardinality of
the ball in the sum-rank metric with that of an optimal linear anticode,
showing that the latter is strictly larger over sufficiently large finite
fields. Finally, we give examples of parameters for which the largest anticode
is neither a ball nor a linear anticode
