I use 20' 4x4 with 3-4 in the ground surrounded by basecourse so the water drains. Just remember to anchor the end of the cables well. I use 4" rebar with a loop welded on top to attach a turnbuckle. Has worked well for me over the last 5 yrs. Remember to note which way the wind comes from and try to aligh the trellis in that direction or you will have alot of stress on the trellis. I have some pics in my profile if you want to look. Good luck!
There is no problem to big or small not to be fixed by enough beer!
Just do not have your beer held hostage by the wife or you will never get anything halfway done!