The only possible answer about a lens is that standing farther away and using a longer lens will be more "flat" than the opposite. Standing close with a short lens would be the worst possible answer. The farther the distance (and therefore the longer lens to magnify it), the flatter the perspective appears.
However, for like a portrait of a human face, 6 to 10 feet distance (a couple of meters) is considered very acceptable for a proper and acceptable perspective view (simply then looks like what we are familiar with seeing). But size still varies with distance, so your compressor requirement (seeking very flat) needs much more distance.
Suppose your compressor is 3 feet tall, meaning if you stand 5 feet away, some parts are at 5 and some parts are at 8 feet. Objects at 8 feet will always be seen smaller than objects at 5 feet. But if you stand 100 feet away, now some parts are 100 feet, and some are 103 feet distant. That would be a very small size difference, which might be called negligible and flat. This is the effect of a longer lens (makes things appear more flat), but that difference is entirely due to the distance, not the lens. But no matter at what distance you stand, there is always that 3 foot difference.
The Rule is: Perspective effects depend only on the distance, NOT on the lens used. That is, perspective depends only on where you stand to view it (i.e., the distance to it). Wherever you stand, all your lens can see is the view seen from that spot. Objects more distant will appear smaller than objects that are closer. That is simply how distance works, and we call it Perspective.
The apparent size is not linear with distance, it varies with with the tangent of the subtended angle, which can be far from linear. Seen up close, the angle is large, causing a large effect. But as the angle becomes small (say less than 10 degrees, the difference approximates becoming linear (however, farther is still always smaller than closer).
To make the angle of size become small requires viewing from a greater distance... which then also makes the size seem small...
so to view it larger then requires the magnification of a longer lens. But the size and angle and perspective depends on the distance, NOT on the lens standing at that distance.
Any lens merely shows what it sees from that location. There is nothing else a lens can do but to show what it sees.
What it sees (the perspective) depends on where you stand to view the scene (i.e., depends only on the distance to the various objects in the scene).
So if you need to show proper proportional views, it sounds like you need to show additional views from more angles (subject objects being at more the SAME distance) to properly show all aspects of size. Maybe also include a spec chart of the dimensions of various parts of it.