This question is a spin-off of this one. Someone asked in the comments, How do we know the Executor is 19km long?
Based solely on canonical (film-only) evidence, how do we know this?
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Below is support for the 19km length of the Executor.
Bear with me, this is a long chain of evidence, but it is all taken directly from movie footage and nowhere else.
The following image is a screenshot from Episode IV taken the moment the Tantive IV fully enters the shadow of the docking bay on the Star Destroyer. At this moment, we can then use this image to measure the size of the Tantive IV and the size of the docking bay.
C-3PO can stand upright in the escape pod, and based on footage
from the surface of Tatooine from inside the pod, the outside diameter of the escape pod is approximately 3 meters.
C-3PO (sitting) and R2-D2 (standing) looking out the escape pod port.
Starting from this point, we get the total length of the Tantive IV as about 120-130 meters (122 in my measurements). The docking bay in the Star Destroyer then measures 192 meters.
There are several other scenes where we can get a very clear picture of docking bay size vs total length of Star Destroyer. My best measurements show the ship is 8.7 times longer than the docking bay, giving a Star Destroyer length of 1672 meters. Just from screenshot measurements.
In ROTJ it is difficult to get precise measurements between a Star Destroyer and the Executor since you are never 100% sure that the two are in the same plane (field of depth). What can be seen clearly is that the Executor is a minimum of 10 times the length of an ordinary Star Destroyer. So, a minimum of 16.7km. 19km is absolutely plausible.
Death Star size is harder to get based on screen footage only, but it is very definitely shown to be smaller than 300km. I currently have no direct evidence to support the previous statement. (I'm definitely not the first one to follow this chain of image evidence.)