Several veins in forearm sting. Veins are soft & skin is normal color. Veins engorge when hands by sides and constrict when raised. Thrombophlebitis?

Not thrombophlebitis. It is normal for veins to expand and contract in relation to their relative position to the height of the heart. Throbophlebitis is a clot in the vein and such a vein would not change with the height of the arm.
No. When veins are at your side, they are engorged because of gravity. When raised, they flatten (veins cannot constrict, they are simply tubes to bring blood back to the heart from the rest of the body; arteries bring blood to the body from the heart, and these vessels can constrict). Don't know what the stinging is.
Multiple reasons. Veins return blood back to the heart and can carry a large volume of blood. Depending on many factors including levels of hydration, genetics, outside temperature and even physical activity, the size of the veins will vary. Another important function of the veins is to help control body temperature and this too will affect vein size. What you describe is not thrombophlebitis.
Thrombophlebitis. Thrombophlebitis causes redness and pain. It is normal for veins to engorge when down and constrict when you raise them. If you have swelling, redness or pain, see your doctor. All the best.