In a construction context, I understand that where two pieces of materials of the same type meet, we call it a joint, e.g. in brickwork or woodwork, between individual bricks, tiles or pieces of timber.
Whereas seam is where two different types of materials meet, e.g. brick and window frame, wall and floor, bathtub and wall ... a seam is typically sealed, e.g. with silicone or acrylic paste.
I am not sure if it is a question of 2D (in plane = joint) vs. 3D (with edge = seam) - anybody knows?
As to the German words in question, Fugenmaße is much less common than Fugenmasse (~ Spachtelmasse, Fugenmörtel). I would not use it to avoid misunderstanding, rather say Fugendimension, Fugenbreite.