There are commercial programs that do this, but of course, you have to buy the program. This shed that you want to build -- it really isn't too hard to lay them out on a sheet of paper and create what you need. But, then, i've worked in that field and laid out a few anyway ... sometimes just dreaming of one for myself.
One important hint is to begin with your outside dimention both ways being a multiple of four feet. Of course, you can use two feet multiples outside. It just means that a 4 x 8 sheet will need to be ripped to fit. Not a problem at all.
The reason is that siding, drywall, etc are usually made in 4 foot by 8 foot sheets. This will minimize your cutting of exterior panels and the interior walls will then have a bit of width (about 7 inches)to remove.
Decide what you want the interior wall to be ... or even if you want interior walls. Depending on where you live, you may want to heat or cool the building. Since you are in Texas, you will want to do both !!! How many doors and windows you want is stictly your personal desire and need. You may, in fact, find it reasonable to insulate the walls.
You can get "window" a/c units (that do not reuire a gunuine window opening to install)at Sam's Club very cost effective. A/c will help you control the inside air humidity for quality painting. An electric space heater should be good for winter -- bearing in mind that potentially explosive liquids you may be using. In fact, "window" units are available that both heat and cool.
A stove hood is very easily usable as your paint booth exhaust system. It can be vented inside or outside the building. Charcoal filters for inside exhaust.
A two-bulb flouresent shop light will provide all the light you are likely to need, depending again on just how big you wnat your shop to be. Is 8 feet by 12 feet big enough ?
Phil, obviously, i could go on quite extensively on this for you. And i will be happy to do that if you need or want me to. Contact me at email@example.com