when annotating the dimensions of an object, i always specify what's what.
ie. 25cmW x 15cmH x 20cmD
W = width
H = height
D = depth
"long" refers to it's width, so the other two dimensions would be height and depth.
without seeing the coil, it's impossible to know which is which.
if it's a circular coil, the height and depth (by definition) would be equal...