1) No real reason why it is so small. The currents on this plane are small, so I only made it big enough to cover all the +5V VIAs and track. I also kept in mind, that GND can be noisy, so maybe smaller power plane may be less sensitive to pick up the noise. But, that is only my feeling, if you like, you can make it bigger, but I would not do it as big as the GND planes because of the 20H rule.
2) I read about VIA fence. I have done this fence on some of my boards, but have not see any difference comparing to boards without fence (both kind of boards pass EMC/EMI oki). I need to say, I normally design digital boards, the fence can make differences in different kind of boards e.g. RF.
3) If this is the one in the middle, there will be only small currents flowing on that part of plane. Feel free to adjust the shape and use any shape you like