the top of your hearth diameter should match your hopper diameter. You can not change physics however you can overcome them.
On the Imbert dimensions chart use specs from one of the first three lines. Pick the one that will best fit your target goal. Do not make any changes to this, use the jet hights, ring diameter, restriction size and jet openings for chipped wood fuels. If you use a flat plate restriction system vs and inverted cone then use the face of this plate to the center height of your jets for the jet height dimensions. Your over all hearth dimensions will be 2" or more larger than what is specified in the chart for these first sets of dimensions in the chart. You can also go larger with your grate diameter, this will allow your machine to breath a little better.
Terminology, ha!! If you were to come to our shop we do not use typical terminology. We look at things much differently. The “hearth”, “fire tube” , “burn chamber” is what we call the oxidation zone our first stage reactor. All stages after this starting at the restriction and throat deal with the reduction and is broken down into different stages of the reduction processes.