The end of any jack regardless of its diameter will not crush the heavy wall of the round axle tube on a std leaf spring suspension. Makes no different the size if the jack against the round axle tube.
A larger diameter jack point is prefered simply because it lessons the chance of the jack slipping.
Now a torsion suspension has thin square axle tubes and there has been reports of crushed tubes from jacking.
Having done my own annual trailer inspections/maintenance on my commercial trailers I would never hook a wheeled vehicle up to a trailer when jacking the trailer off the ground. Not if I have to work under there.
Russ' s pictures tells a good safe story. I have the same type cribbing only their 2x6 blocks. I also have lots of 24"-30" long blocks made from RR ties. These can be placed under the trailers frame rails at angles to each other. This keeps the trailer stabile even in high winds.
Attention to the length of the trailer is required.
One jack placed under the main frame rail behind the last spring hanger to lift the trailer works on shorter trailers.
Two lifting jacks.... one somewhere behind the aft spring hanger and somewhere forward of the front spring hanger works better on longer/heavier trailers.
Think safety before each move. When you think its safe to go under walk back to the rear corner of the trailer and push hard to see how stable its sitting. If it wiggles add more blocks or relocate blocks till its stable. Good luck