It varies to be honest.
For longfin especially it is better to use body length rather than total length because as you mention you have the long tail to contend with.
so total length goes from the tip of the head to the tip of the tail, body length is normally the distance from the tip of the head to the start of the tail fin, although, for some (generally scientific purposes) body length means tip of the head to the anus.
What we refer to size it is generally total length of a short fin or body length of a long fin.
Does that help, not sure if i explained that well enough haha
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