and that takes a good long while to break down compared to grass clippings which are mostly nitrogen and break down quickly.
Shredding leaves and mixing them with grass clippings is the best way to compost them.
When leaves just lay on the ground, worms and bacteria devour them quickly. Piled up, not so much.
They will decompose but it may take a few more years. Just keep em wet.

It can be any size depending on the space you have. We had just one for a long time, but with 5 gardens going, we made a second bin. Each is about 4X8 blocks in size, about 4 ft. high. Believe it or not, they get filled, too.

We throw leaves, mowed grass (don't allow neighbors to add in case they put chemicals on their yards), water melon/muskmelon rinds, corn cobs, and garden waste into it. Mr. Shraby waters it from time to time and when we find night crawlers/worms that come out to the pavement after a rain, we toss them in the compost.
After a couple years we open one, take the top off down to the black dirt and use the dirt on the gardens, then put the top back in the bin. The next year we use out of the other one.

I don't have to dig very far down into my biggest horse manure pile right now to hit large amounts of black gold. I've never turned it. But it gets combined with wet hay and pine shavings from cleaning stalls. And its totally in the open.

And by spring the bottom 2 feet or so is leaf mold. It rains a lot here, so they have an advantage over other parts of the country that don’t get as much rain. The worms do their magic but it’d take another year to get to the top two feet of the wire round. I tend to use those top two feet of not-quite-leaf-mold in my garden beds in the next fall, or even in the first spring if I’m building a new bed.