1. Specific to me, I only had this 8x8 space I felt was reasonable to dedicate to the pond. In retrospect, I wish I had gone 8x12, but isn't that the case with everything? As-is, it's a very good visual balance in the patio, so I should just be happy. Regardless, so with 8x8 (8x8.5' to be exact), every additional foot gave me another 500 gallons of volume roughly, so by going to 6' instead of 4', I got another 1000 gallons. Water quality is really important to koi health and more volume per fish slows down chemistry changes that could otherwise get out of hand quickly. The general rule of thumb is 300 - 500 gallons per koi. The bigger the fish, the higher that ratio goes. It also depends on your filtration, etc... In the end, that extra 1000 gallons meant I could get a couple more and/or larger fish.
2. The thinking in the koi world is that deeper ponds help with koi growth and health because they have to work more to change depths. Nature has tuned them into a fairly low-drag form, so scooting around a pond is really not much exercise for them. Changing depth however requires a bit more effort.
The heat is not really an issue here. In fact if anything it helps extend the "growing season" when you can really try to bulk up the fish and get them to grow by feeding heavily when your water temp is in that 70(f) to 80(f) sweet spot. The extra volume does help slow any rapid temperature changes which is a plus. You see ponds all over here though that are as little as 2' and in direct sun. I am sure they get up into the high 90(f)'s in July/August which is past the upper healthy temperature limits. Those fish survive just fine though anyway. This pond is 100% out of direct sunlight in the summer. Autumn - Spring it does get some direct sunlight when the sun is far enough south in the sky to shine in under the roof.
Yes, the pond is my "zen" area. As long as it's not cold (mid Dec - Feb in Houston normally), I am out there 2-5 times a day (I work from a home office) to just sit a little while, relax and feed the fish who now all come up to beg for food and eat out of my hand.
So here is something that is kind of fun. I had our security camera set during the beginning of construction to take a snapshot every 15 minutes from about 7am until 7pm. I strung all of these together as a time-lapse. The quality is not super great, but it does show how things happened. The time covered is August 22 through December 10 and is made up of approximately 3300 individual photos.