Any place that sells batteries can tell you the correct size to buy.
Mercury 1984 6 cylinder L6 what type & size of starting battery is required
550 cca
The 75 hp Mercury outboard will need a starting battery in the 650 CCA range, although, up to 1000 CCA would work just fine.
For a 2005, Mercury 3.3 hp outboard, a NGK BPR6HS.
Mercury wants you to install a battery with at least 525 marine cranking amps if you boat in the summer, and 1000 marine cranking amps for winter boaters.
1 gauge cable should do the job
For a 2 stroke model, a 500 to 650 cca, 12 volt starting battery will be sufficient. The recommended battery for a four stroke model is 1000 cca,
OD281588
For a 150 hp Mercury outboard, a 12V battery with a minimum capacity of 800 CCA (cold cranking amps) is typically recommended. A group size 27 or 31 battery is often suitable, as they provide adequate power for starting and running the motor. It's also advisable to consider a deep-cycle or dual-purpose battery to support additional electrical systems on board. Always check the manufacturer's specifications for the most accurate recommendations.
Depends if it fiberglass or Aluminium. For Fiberglass you would be looking at around 50hp-60hp and and Aluminium would be between 30hp and 40hp.
1-1/2x16
36R