Cluster sizing - Query and Index nodes

I’m going through the cluster sizing document put up here:
…and I had a couple of questions:

In the exampled given under ‘Index Service Nodes’ it says:
Secondary index 24 bytes

Q1: What does this mean?
Q2: Based on the number of fields that need to be indexed, how can we estimate the size of the index, if at all?

Q3: Is the memory and disk spec of Query nodes important while sizing? Or do we just care about the processing spec?