First off, a parry isn't a block. You don't stop an enemies attack cold in it's tracks when you parry. For one thing, outside of hollywood, stopping an enemies swinging sword dead in its tracks is impossible to do in most cases. Rather, a parry is a deflection of the incoming attack. You are knocking the sword to the side, where it isn't hitting you anymore.
Parrying a dreadnought though IS a difficult situation to answer. According to the rules, you can parry any melee attack, but its up to your GM though to rule on these situations, to use common sense or even real world mechanics to determine, as the Core rulebook says, "When adjudicating difficulty, common sense should prevail. Regardless of the usual limits on test penalties, some Actions are simply impossible (DW, p.244)."
As for size, according to the rules question fellows, there are no size modifiers to melee attacks. This includes parrying, you never claim size bonuses when making or evading melee attacks.
A defiler is a neat enemy. As for parrying, keep in mind the previous paragraphs. As a GM though you have to keep in mind the characters (the defiler in this case) abilities, what actions are available to them, and what the character might do in any situation.
First off the defiler has the lightning attack talent, this allows it to make 3 melee attacks with a full action. Thats a lot of attacks.
Second a defiler may both shoot and melee in the same turn. It may shoot ALL of its weapons as a single half action (it can do this even if it isn't meleeing as well) and then is able to use another half action to make a single melee attack.
Third, a defiler's weapons seem more geared towards vehicle mayhem than infantry killing. When trying to punch tanks I'd give the thing some hefty bonuses. I mean a tank isn't agile or weavy as an enemy soldier would be. I'd at least rule a tank is Unaware, and if the vehicle isn't even moving then I'd consider it a helpless target, for melee at least.
Fourth, a higher WS doesn't benefit anyone if you are grappled. I'd imagine a daemonically possessed engine of death would take just as much pleasure of slowly crushing the life out of its target as smashing it to a pulp. A defiler has weapons, but like a power fist, these are also hands so they cross the lines of being both unarmed (for grappling) and armed with a weapon. Plus, according to the rules fellows, you may use grapple attacks with Swift attack (and most likely lightning attack as well). Which means the defiler gets 3 chances to start a grapple against an enemy in one turn. Or gets to shoot all its guns and 1 chance to grapple. Once grappled the difiler is s75 (USx2), one size level larger than a space marine, and counts its full claw damage as unarmed damage. That means it gets 3 extra DoS (2 for US + 1 for being bigger) to the space marines 2, and does a ton of damage. I'd even rule the defiler could start a grapple in one turn, maintain it as a free or half action, allowing it to pick up and carry around its grappled target while it continued to smash others in hand to hand with its other arm (it is daemonically possessed after all).
So the defiler can be a very dangerous opponent in hand to hand combat, GMs just have to use a little creative thinking sometimes.