I used 8x8 treated.. by 16 inches.. attached ropes, end to end.. as handles.
the less you need to extend your jacks.. the more stable the trailer is.. when walking inside trailer.
my opinion..
and I am OLD...:)
I purchase the warped/damaged posts from Home Depot.. 80 percent off price. :) so real cheap... but I am there once a month anyway..
140 year old Home always needs something fixed/done.. and the wife to-do list.