I think that Marcus is selling plain 6TNs, which are conventional flooded lead-acids, as opposed to the 6TNMF (Hawker). They won't last as long as the Hawkers so may prove a false economy. All 6TN variants should be the same size, 286mm x 269mm x 230mm. There is a version without the breather manifold (as used by the US) but it isn't any lower as the overall height is dictated by the terminals.
Andy