Even though the container had some vents I wanted a little more. Since they are under the lean too I didn't need to louver them. 6x12 should do the trick.
Hot really wasn't the issue. I was more concerned with moisture. A few times I opened it up and it had some condensation on the ceiling, not a lot but some. I do plan on another near the bottom but didn't have time for it today.
Since I have it under the lean too I wasn't to worried about it. We did get some wind and rain the other day and I had a little get in. I plan to add a ridge cap on top of the opening. I have also added a larger one on the bottom to get better air flow. Also put a window screen between 2 of the larger wires screen to keep the bugs out.
Mine also cooled down considerably. Condensation is also not a problem. I keep the wheeler, chainsaw and tractor in mine. Along with fuel for each, no problems with fumes.
I have 4 chainsaws, gas, oil, diesel, the tractor, weed whacker (well, brushcutter) - the tractor will smoke you out in a hurry running it, but it clears out quickly. Doesn't smell of anything when I open it up.