There is more than Internet research in preparing for scratch build (for those uninitiated). Here is an example of what I do to get an estimation of size. My first goal is to find a reference point. Human Factor Engineering points to average sizes of a human. In this model I sought the tallest man and marked him as 6 feet. He was already on a bridge photo.

My goal is NOT to be exact for the whole bridge. But the reference ought to be as close to exact as possible, so other photo measurements (pixels) will give a more accurate estimation. Below is an example of getting a feel for the timber sizes, and to find out about bridge width and railing height.

Slight variance in estimation is required due to perspective/depth from point of reference (A.Guinness)
This image was reduced to fit in forum.

Crikey! There's a lot of crazy mixed impressions from the collected photos, due to shadows and mixed timber widths. Cannot escape from the fact that the main uprights lean inwards to the top, in both width and length. It's almost forcing me to scratchbuild the bridge in situ, as if building a real one.

I am tempted to do my best _ yet keeping it simple. Will lean in the uprights, but most of the joints will be altered to a simple manner. That is, not to engineering prudence. I am doing it this way to lessen the impact of self-inflicted insanity.

Spent many hours of close examination of the movie bridge. The real bridges and trusses along the Burma railway was built a lot better than the movie bridge. It became obvious that the movie bridge builders had 'different priorities' to normal bridge construction methods.

1) It was to be blown up after construction.
2) Had to hold up a train with wagons.
3) Look impressive where it counts (from afar, and along the tracks). The rest did not matter.

Even our own small wooden creek/brook bridges were built far better than the movie bridge. Purely because it have to survive at least ten years.

Just for a point of interest:

Hans @wimfi, thank you for visiting and relating that observation. Yes, I agree with you. Have not updated the drawings to reflect how the main uprights lean inwards. They do so from both sides (length and width).

Okay, I worked out the base dimensions for the diorama. It's a nice size that will give an overview of the bridge and its construction. It is large enough to add small figurines and small enough to fit on a bookcase-shelf.

CORRECTION. Base width is now 160 mm

Thanks Paul. I will do my best.
Keeping the reality of the situation in mind will keep me on track.

Last year, I had lunch with one of the Aussies. He loved his fish and chips with lemon. Apparently he always ate the lemon rind too. He never wasted any food, even after all these years. He died from old age (in his 90s), not long after. As much as he kept it in mind, I hope to do the same during this build.

Had a go at whitling 1/2 inch figurines. Not that easy. Definitly need a sharp pointed x-acto blade.
What was not easy was finding how far to go. Whenever I thought I trimmed the body down enough, I took a photo. Then comparising photo with to actual survivor photos, I soon discovered I had a lot more to trim off. I had to repeat this process a few times. It really made me realize how much these guys suffered _ dying from starvation and exhaustion.

Building this model is strange for me. I am not in therapy, but I may need debriefing by the time I finish.

Thanks David for your concern. Much appreciated.

Will leave it at the modelling table, just like I used to do at work. As soon as it was time to go home, it was ALL left at work. I am not responsible for others, only responsible to do my best. After twenty years, I never had burnout. But that does not mean, while with the client, I did not cry in empathy for their grief and loss. There is a time and place for everything. If I need a debfief, I'll have a chat with a colleague in town. You never know, I might get him to create models too.