Looks like I'm going to play my 'butt in' card once again, but not for rules or tactics. I happen to play both armies IRL so I think I can help you out here. Tau Devilfish chassis'(Devilfish, Hammerhead, Skyray) are slightly under 5.5'' from nose to the back of the hull., and about 5'' from engine to engine. Possible burst cannon 'sponsons' not taken into account.
As for Dark Eldar Raider chassis'(Raider, Ravager), it is 7'' from tip of prow to the back edge of the rudder. At the widest points(the flared parts) they are 3'' wide, and the Ravager sponsons don't alter that.