How can you know the size bars needed if the trailer is old and weight information is not available from the manufacturer and you have not physically weighed the entire rig?
You could try to guess by looking at information for something similar, but then,,, you are still guessing.
Find a scale
Weigh your tow vehicle alone.
weigh your tow vehicle only with trailer connected but off the scale.
weigh the entire rig.
Then you can decide on the size bars you want. Until you have some basic info to work with, be it from a manufacturer or a scale, you are only guessing.
Edit add;
Maybe this will help with what you are asking.
https://www.youtube.com/watch?v=anhi8lo4Od0
But you still need to know the weights to choose the appropriate size bars.