Student or Learner
His tail stretched out along the floor behind him for a good yard and looked like a fat black snake.
it's something like a half metre?
So, Tinkerbell, 'a good yard' is more like a metre than half a metre. Generally, 'a good' can be used before any approximate measurement or assessment: a good inch, a good spoonful, a good sprinkling of snow.... It can't be used before a specific measurement - a good 1.358 grams ( )