Minimum field size.

Big subject covered in the Glider Flight Training Manual beginning on page 248 through page 256.

Clear the trees by a safe margin. In this case round it off to 100 feet.

Full dive brakes (type certified glider) and no wind, the glide ratio is 5:1.

This results in 500 feet plus round-out, flare, touchdown, apply brakes and stop.

Being the expert pilot you are, make it 1,000 feet. Try it at your familiar home airport.

Tom Knauff

Lots more info in the book.

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.