Why are TE probes so long?

Yikes, my head hurts!

Static pressure is simply that. The static pressure of the ambient
air around you. A good aircraft static system should give you the
same value for any given ambient pressure (ie altitude) no matter how
fast your glider is going.

Dynamic pressure is the pressure due to velocity and it varies by the
square of the velocity (ie twice the velocity = four times the
pressure). The ASI measures the difference between the dynamic
(pitot) pressure and the static pressure to give velocity.

Keep in mind that the static pressure at the pitot and at the static
port are the same and cancel each other out in the equation (unless
like most of us you have a horrible static system).

The difference between the pressure at the TE probe and static
pressure should be the same as the dynamic pressure but of opposite
sign. Again static pressure at both locations is the same so it
cancels.

I have driven many miles with an automobile test rig consisting of a
pitot, a static and a TE probe connected to a pair of side by side
U-tube water manometers. The manometers share the same static and one
uses the pitot signal while the other uses the TE signal. A properly
calibrated TE probe will cause exactly the same pressure difference as
the pitot, just with the opposite sign. The mounting bracket for the
probes hung out way in front of the car to get clean air for the
testing. I must say it got quite a few stares.

In practice I usually build TE probes to overcompensate slightly to
make up for poor static systems. The probes are easy for the user to
tame down slightly and they can be tailored by the individual pilot to
match their preference and sailplane.