I think the size estimation could also be implemented within the provided abstractions (mainly request-response) but might require you to keep track of neighbors. I think you could implement both algorithms by using our SDKs (none for Go yet).
If you need more control or performance, beyond what we expose through our SDKs, you might need to write a custom libp2p behavior and add it to our daemon. The libp2p part is fairly involved, but I would love to help you with that. Either way I would love to help you out :)
I'm so disappointed that I've never seen your blog before. The stuff you write about is so interesting and actually addresses some issues we are facing. I just sent you an email :)