The smaller the tag the fewer biological ramifications or effects. Tag loss occurs less often. But small tags have limited read range and which restricts the size of the antenna that can be used. Shorter read ranges can also decrease reading ability.

Studies of salmonids in the Columbia River Basin in the northwestern US suggest these minimum fork lengths by tag size:
60 mm -- 12 x 2.15 mm
100 mm -- 23 x 3.65 mm