Will they keep getting bigger and bigger?

I recently installed this graphics card in my brand new Dell XPS and it BARELY fit. It has been seven years since I upgraded video cards and I remember them being near half the size of what they are now.

http://www.newegg.com/Product/Prod [...] 6814102948

Will video cards continue to increase in size to the point where they won't fit PC's like the Dell XPS?

I don't believe they will. The largest card currently available is no larger than the 4870 x2 which was released in 2008. Width-wise, they shouldn't get larger than 2 slots with the exception being special units by companies hoping to charge more for 'better cooling' units. The reason for this is that it retracts from the number of other slots of the motherboard, creating an undo diminishing return in performance vs. space taken up.

There are set standards for the sizes of PCIe cards, outlined by PCI-SIG. If a card is outside of these specifications, it can't be called PCIe.
It's up to the case manufacturer to follow this though...

I highly doubt it. They are starting to hit the point where each new generation is able to have slight to moderate gains in performance but substantial gains in reducing power consumption and voltage.

As they continue to become more efficient in those area's they should also run much cooler and require less shrouding for cooling which is one of the main reasons the cards like you linked are double the width than what you were probably used to seeing. I can see some of the entry level models even going back to single width designs.

The enthusiast level cards may be a whole different story, but I can't see enthusiasts suddenly changing from custom building their PC's to purchasing something like a Dell XPS where they have to worry about size of parts and layout either