My question is,
how long a wire should I use to have a good shortwave antenna that will fit
in my yard.

Two things. Firstly, depending on the radio you're using, you may not want even
86 feet of wire, since it may cause overload on stronger signals. With a
portable, 50 feet might be too much.
Secondly, ANY amount of wire that is up as high as you can get it but away from
powerlines (for safety AND noise/interference reasons) will be a good antenna.
86 feet is a lot to work with, so consider yourself lucky, see how high you can
get it up there, and have fun experimenting with antennas! :-)
Linus