Dual battery wire size and mounting

Working on a dual battery setup and was wondering about the wire size. I have the 2.4l 3rd gen, which I believe has a 100 amp alternator. Should I size the wires running from the start battery to the secondary with the full 100 amps in mind? Or is the actual current flow lower?
Also, does anyone have any good spots to put a second battery? I was worried if a mount in the engine bay would hold since it’s heavy

 

If you're not sure, go with the larger wire. Use a chart like this to figure what size wire to use.
I have a

$81 Smart Dual Battery 140A Isolator - Auto/Boat/RV, Model: VSR12, Car & Vehicle Accessories / Parts

voltage sensing relay that closes the circuit when the truck is on, and all of a sudden all that amperage flows from the starting battery to the auxiliary battery, so plan on that.

I also didn't want my aux battery to be under the hood because it is cramped so I mounted it under the truck bed, above the drive shaft. I haven't had any issues with it in 7 years, and transferred it from my 1st gen Tundra to my Tacoma. I made a video about it back then on my Tundra.