I agree with Corey, you do want to protect yourself to some degree so that it's not cranking itself up to 120 degrees or more if something fails (or in the case of backup thermostats two somethings fail)

I've seen 11 inch at 145 degrees.

I would suggest running 3 pieces of either 3 inch 6 watt or 4 inch 8 watt (the 3 inch is probably better IMO). A piece up the back and a piece up each of the sides. Pick the length that gives you the best results for good heat distribution and a reasonable max temp.

Then add a fan or two to create good circulation (could do one side blowing air upward and a fan on the other side blowing air downward)

Ultimately you have to play around with things to see what works for you and your incubator.

As far as backups.....I use a proportional thermostat as my main with the probe placed in one of the egg containers on the middle shelf and set it at 88.5 degrees. I then hook that into a backup on/off thermostat set at 91 degrees and place the probe outside of the egg boxes, in the "hottest" part of my incubator, in the ambiant incubator air (for me this is the bottom row where more of my flexwatt is).

This helps so that if the proportional is working hard to get the temps back up that the backup shuts it off so that my "hot spot" doesn't get too warm and it allows the temps to raise gradually without overworking the heattape.

I think there are also settings on the spyder robotics thermostats to change the temps gradually but haven't played with this myself.

Hope I helped Best of luck!