Whats the best wood size, and type for hugelkultur?

So I'm wanting to install a few below ground hugelkultur beds on my property and I'm wondering what the best type of wood to use. I remember hearing on one of the videos that pine and conifers are less desirable than other types, why is that? What kinds of wood are best for this practice and why? Is there any kind of wood you shouldn't use (like really oily wood like Eucalyptus maybe? ) I'd really like to understand more about this.

Also, I keep seeing people use giant stumps and logs. Is there a preferred size to use? Do giant logs work just as well as the same amount in branches and dead fall? What about sawdust?