What is something that is about milligram long?

A milligram is a unit of mass, not length, so it doesn't directly correspond to a physical measurement like length. However, if we consider something that weighs about a milligram, it could be a grain of salt or a small paperclip. In terms of length, a millimeter is a more appropriate unit, as it is a measure of distance where 1 milligram of water would occupy about 1 cubic millimeter.