Minimum field size.

I will add, look for powerlines in the area.
I was at a central PA contest that had lots of off field landings one day. One pilot was coming into a HUGE harvested field (I did a retrieve from the other end that same day) over a tree line and house. This was going into Altoona, PA.
He was low over the trees, but NEVER saw the power lines that went through the tree tops.
End result, he hit a top line, it slid under the nose to the main gear and pitched him straight down from about 60'.
Broken glider, broken ankles, took a few years before he flew solo again.

As stated before, do you practice "spot landings" on pretty much any landing?
Have you done "max performance" decents recently?

What some replies make seem simple, others will be nowhere close.