Design notes for Proclamator-class:
Some basics: overall length 889.6m (hull), power ~1.5e24W
The basic concept of this ship was as a full warship equivalent to Acclamator, in the same design lineage.
In my background for the design, this was basically an Old Republic ISD: a flexible ship suitable for many roles and independent action with its own troops and fighters. Before the expansion in ship sizes during the Clone Wars, this would probably have been rated a Star Destroyer. The design is a good one, so it just gets re-rated and continues long in service into the Imperial era.
In my interpretation, when it came time to make a fast assault ship, the designers took an existing warship hull design and stripped it down, like a razée. Since it’s easier to remove mass than add it, and keeping costs down is important to get numbers quickly and replace losses in contested landings, the resulting design lost a lot of warship capability by removing heavy (and expensive) things like heavy weapons, armor, and power systems. But the loss of weight meant that a lot of the speed is retained (see the related concepts expanded in: Star Wars Ship Design Technical Notes 1).
So take the Proclamator’s hull, and fit a much smaller but much more efficient reactor to it for range. All the heavy turbolasers and their structural bracing and power systems go, and the medium quad turbolasers are redistributed a bit. Cut down the fantail and a lot of the ventral hull to remove armor weight, and reassign hangar bay volume to internal troop and vehicle storage. All the savings in weight in armor and structure means engines can be down-rated as well. The inner pair of main engines are retained, and the middle pair reduced to secondaries (the engine axes are the same relative position as in an Acclamator). Outer engines are removed, saving more weight and cost. I have purposefully kept the hull lines very similar to Acclamator at least in plan and dorsal profile – the differences generally follow direct extensions of Acclamator profile lines, except the ventral and engine areas. As a combat transport, warship sensor and command spaces are less important, so the bridge module and sensor fit gets cut down a lot. In general, Acclamator would be this design, trimmed of a lot of excess armored volume.