I work for ShoreLand'r boat trailers and I can tell you that we ALWAYS
recommend crossing the safety chains.
I learned why the hard way...

A few years back I was towing my boss' boat on a bumpy 2-lane
highway. I hit a dip at 55 mph and the trailer popped off the ball.
Trust me on this one - when that happens, you know immediately!
I pumped the brakes to slow down my truck and the trailer sounded like
it was ramming into my bumper. There was a loud snapping noise and in
the rear view mirror I could see the boat swerving wildly from side to
side. I was sure it was going into the ditch! I kept pumping the
brakes and was finally able to bring the rig to a stop.

When I walked around to the back of my vehicle I was amazed to see how
well the safety chains had worked. They had caught the coupler just
as intended, but they hadn't been shortened up enough and all that
grinding on the pavement had worn one of the chains thru to the point
where it had snapped - that explained all the wild swerving.

In the end we found out what had caused the coupler to come unhooked -
the hitch ball was too small for the coupler. I hadn't noticed when
I hooked it up, but I sure watch for that now. And I ALWAYS remember
to cross my safety chains! Another feature that saved my butt was the
tiedowns - they kept that boat secured to the trailer so it didn't fly
off into the ditch.

As for the question about the proper length when you are twisting the
chains to shorten them up... safety chains come in a variety of
lengths, so there is no exact measurement. The rule of thumb is to
get them just short enough so they aren't dragging on the ground. You
don't want to shorten them too much or it will cause problems during
sharp turns.

If you guys have any questions, don't hesitate to drop me an e-mail...
Mike Melone