Best way to determine size is to drain the tank, then fill it slowly, 3 or 6 gallons at a time, adding oil as you go.
Level the boat on the trailer before filling it, and keep an eye on the fuel gauge, making note of how much gas you add, and what the reading is on the gauge. This will come in handy some day, knowing that there is 6-?? gallons left in the tank when it reads 1/4.
To drain tank:
Tilt tongue of trailer as high as possible, or park on an incline.
Disconnect fuel line from motor, remove fitting from line.
If fuel line is long enough, hang it over the transom and get the end below the bottom of the hull.
It the line is not long enough to do this, buy a few feet of fuel line and a barbed hose fitting to connect the two.
Place end of hose in a fuel can or clean 5 gallon bucket.
Use the primer bulb in the fuel line to start the siphon. It will be slow, and may stop occasionally, but should work.
It may take hours, monitor the bucket so it doesn't overflow.
An owners manual will likely do you no good, they are not like manuals for cars, just have generic boat info.
