1. For a 18" female, a 27 qt Sterilite would be big enough but if you make the bp feel really safe you can use the 41 qt Sterilite. I use 41 qt sterilites for my bps. Here is a link to the sterilite site with the info on the type of tubs that I use.
http://www.sterilite.com/Category.ht...uctCategory=39

2. As long as the flexwatt does not cover more than 1/2 of the enclosure bottom. I use 11" x 12" strips on my 41 qt tubs.

3. Big enough so your bp cant tip it over. All that 'it should be big enough for the bp to soak in' is not really necessary. If the ball python is healthy it will never need to soak.

4. Yep there are clear ones...they may have a slight haze but they are clear...check this one out...it is the old 90 qt setup that I had:
http://www.ball-pythons.net/gallery/...1_original.jpg

let me know if you need more info......here is a DIY plastic housing article:
http://www.ball-pythons.net/modules....warticle&id=40