Big logs or small sticks and twigs?

Does the wood in the hugelbed need to be big logs or can it be lots of brush and sticks? Does this affect the moisture retention or anything else? I just started one and have been throwing sticks, paper, food scraps our animals won't eat, grass clippings and anything else organic in it before I cover it with dirt. This one will be 3 feet in the ground and 3 feet above.

Twigs and logs serve different purposes in the hugel. The twigs have a higher ratio of cambium/bark layers to the dead inner wood, so more nutrient dense and faster rotting. While the logs are less nutrient dense and will rot slower, requiring the work of lots of fungus to release the nutrients from the cellulose and lingnin, also acting as large spongy water reserves in the process.

If it is all brush and sticks I don't think it will be any less effective. Maybe there could be a little N loss due to the twigs rotting too fast, but it can't be worse than planting into a compost pile. The rotted twigs will still hold water too. But I would imagine since they have large surface areas for their size, they will not hold it as long as compared to a few large hunks of rotten wood.

Regardless of whether or not it's a cookie cutter hugel-style, you are creating a better soil than you started with.