There is a sign for the concept of "VERY." It looks like the sign
"BIG" but is done with "V" handshapes.
But I suggest you not use it and instead focus on
inflecting your signs to
include the meaning of "very" in the sign itself.
For example, suppose our friend got sunburned badly and I wanted to tell
you about it, I might wish to express the concept:
"His face was very red."
In that sentence the word "very" is an adverb. The word "red" is an
adjective.
In ASL I'd use the signs: "HIS FACE RED." To indicate the
concept of "very red" I would "inflect" (change the way I signed) the
concept "red" in the following ways:
1. I'd use an intense facial expression
2. I'd hold the initial handshape in starting location for a fraction
of an instant longer before starting the movement.
3. I'd do a larger downward movement.
4. I'd hold the ending handshape in the ending location for a fraction of an
instant longer than normal.
5. At the beginning of the sign I'd tilt my head back slightly and then as I
did the sign I'd nod my head using a single, short, quick movement.
6. My elbow would stick slightly farther out to the side at the beginning of
the sign and bring the elbow down sharply during the sign.
Those six modifications (inflections) to the sign "RED" would change the
sign to mean "very-RED."
Those six modifications actually created the adverb "very."
Most verbs and many other signs can be inflected in such a way as to
eliminate for need for a separate sign for "very."