Few quick questions (heat pads, hides, and substrate)

[Wiggy]

First question, heat pads. My enclosure is 24x24. I know the general rule of thumb is that the pad should cover about 1/3 of the bottom. So this would require a 192 square inch pad.

The 2 pads I'm looking at are a ZooMed 8x18, or Exo-Terra 11x17.

The ZooMed I understand are easier to remove (if I ever have to). Also, my thinking is that while the Exo-Terra, on paper, fits this 'rule' better, the narrower ZooMed will allow the snake to move a farther distance from the actual heat source, so a better temperature gradient could be made.

I'm sure in practice I'm probably overthinking this a bit, but might as well get some opinions before I place the order.

Second question, hides. I understand that a hide should be just big enough for the ball python to fit in and feel snug on all sides. Not having an actual python though, it's hard to guess how big this would be. Could I get some rough dimensions to use as guidelines?

Final question, substrate. I plan to use Aspen. The site I'm ordering the heat pad from does have aspen marketed as a substrate for snakes. The Pet Value nearby has Aspen for about half the price in larger bags.

Easy Clean Aspen
http://i.tfcdn.com/img2/vN9_-EAAY1Jd...kAwA*/fyVMtP8A

Says it's good for gerbils, rabits, mice, and other small animals.

Does anyone know if this stuff is suitable? I was expecting just shavings, but this had some 'chip' pieces in it. I guess I could always pick out the chips and just use the shavings?