I have been stocking up on fish and since you can reuse the bags I think it makes sense to cut them an extra 6 inches or so longer than you need so you can get more reuses out of them.
Does this make sense to the vacuum sealing pros?
I find that the thinner fish like trout, king makerel, gar, work much better with the purple Dyson.
For the wider fish like flounder, haddock, skate work great with the old Hoover.
Are you freezing ahead of time? Keeps the liquids back.
On most items I give it 1-2 uses and don't reuse on fish. I only leave 3"
If you buy the good bags,( I don't,) they can go a couple of times.
I get mine off ebay and they get kinda brittle after 2 uses.