The spell enlarge person says, "Multiple magical effects that increase size do not stack." The spell righteous might says, "Magical effects that increase size do not stack." The bloodrager's abyssal bloodline special ability demonic bulk is a supernatural ability (hence magical) that's based on the spell enlarge person, so it won't stack twice.
Also, the FAQ includes this exchange:
Question: Size increases and effective size increases: How does damage work if I have various effects that change my actual size, my effective size, and my damage dice?
Answer: As per the rules on size changes, size changes do not stack, so if you have multiple size changing effects (for instance an effect that increases your size by one step and another that increases your size by two steps), only the largest applies. The same is true of effective size increases (which includes “deal damage as if they were one size category larger than they actually are,” “your damage die type increases by one step,” and similar language). They don’t stack with each other, just take the biggest one. However, you can have one of each and they do work together (for example, enlarge person increasing your actual size to Large and a bashing shield increasing your shield’s effective size by two steps, for a total of 2d6 damage).
If you're willing to wade through 200+ posts on this topic, you can do that here, but, in short, the FAQ ruling means that you can have running simultaneously the largest actual size increase and the largest effective size increase. For example, a Medium human that's been the subject of an enlarge person spell and whose greatsword had been the subject of the spell lead blades has a greatsword that deals 4d6 points of damage sans modifiers (like a Huge greatsword does). However, I think were the same enlarged human to activate his +1 growing greatsword, the greatsword would deal only 3d6 points of damage (like a Large weapon does), the +1 growing greatsword already having had its size actually increased. (Nonetheless, while active, the magic weapon special ability growing would allow that enlarged human to use combat maneuvers against creatures he normally couldn't, so it's not a total loss.)
The only way I'm aware of to instantaneously increase a creature's size (i.e. immune to dispel magic and lasting forever) is by drawing the 5 of diamonds from a harrow deck of many things.