There is really no such thing as a perfect sized forehead. A forehead must fit your face. A forehead that might be a good size for your face might be too large for someone with a smaller face or too small for someone with a bigger face. A forehead can also be used to provide a short face with some length or a long face with a reduction in length.

To decide how much I lower a hairline or reduce the size of the forehead I try to make the forehead match the rest of the facial features. This is not a cookie cutter approach. The resulting forehead should just fit your face and it should not be obvious that anything was done. If your forehead is too big for your face it is leading to an imbalance in facial features. When the facial features are rebalanced by lowering the hairline the face should just look in greater balance.

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The face (except for the ears) rarely forms keloids after surgery. Even when there is a history of keloids on other parts of the body, it is still rare to form a keloid on the face--rare but not impossible. Keloids are different than hypertrophic scars, which can form anywhere on the face as well. Unless you have a history of forming a keloid on the face, I would not worry about a keloid forming after surgery on the face.