look at it from another standpoint.
what is the usage? Do you jump in the boat, and run for 10+ miles one way full throttle? Are you using a 2 or 4 stroke? What size? And lastly and just as-if not MORE imporantly-how long does it sit between uses?
If you make long runs and/or are using a bigger 2 stroke outboard (25+hp), 6 gallons. Maybe more. Try to keep an eye on your fuel level as you are using it the first time. Maybe you can borrow a tank to try before you buy one?
A 4 stroke motor uses roughly half the fuel a 2 stroke does. Typically at full throttle full speed full load, a 4 stroke 25hp will use between 2 and 2.5 gallons per hour. And trust me when I say, 1 hour at full throttle full speed is a LONG time in a tiller steered boat. A 2 stroke 25 will use about 4 gal/hr give or take a half or so.
If you use your boat a weekend, then let it sit for 2-3 months, it won't matter what size tank you get because you will be draining the old crap out pretty often anyway. Use a 3 gallon. That way you don't have to throw away 6 gallons of liquid gold. One thing I found with my stuff is that I never know what my schedule is from day to day, so I only keep 3 gallons....when I do get to go, if I use up 3/4 of a 3 gal tank, and let 1/4 of it get old, at least the sting of having to throw it out (or put it in the mower, whatever) is a little less.
weight is another concern. IF you don't need 6 gallons, carry 3. Half the weight over the stern. Or actually probably a little less than half since there is less material in the smaller fuel tank's construction than a 6. Splitting hairs there but you get the idea.