Picture of Custom Reptile Cage
The size of the enclosure is all up to you. I built this one for my 4 month old ball python named Marley, I designed it so that he could grow into it and it would still be a large cage for him. Before I built this he was in a 10 gallon tank and he loves this cage. this one is 30" W x 26" D x 10" H. 

Dimensions of the cage: The cage can be whatever size you would like it to be, but keep in mind what kind of reptile you have. ball pythons being ground snakes there was no real need for the cage to be very tall. I had plans to build a cage for two baby bearded dragons so they could grow into that one and I figured that a cage 14' high was the best. Included in the bearded dragon cage design would be expanding foam ledges (discussed later). If you would like the cage to fit into a certain space be sure that the cage including the supports can fit in that area.

-Table saw (or circular saw)
-Jigsaw with a plexiglass cutting blade, along with a standard wood cutting blade 
-wood screws 1 1/2"
-screws 3/4" and 1"
-pine wood 1"x2" and 1"x3" (comes in 8' and 10' sections at Home depot)
-2"x2" for supports
-Plexiglass .093" thickness
-plywood about 1/4"
-screen clips 
-1" hinges
-1 1/2" bolt latch 
-Mesh screen (screen door screen) optional
-white pegboard
-hot glue gun
-staple gun
-any stain (your choice I used the small can and was able to stain the entire thing with about 1/4 can left over and i used woodstock stain)
-mold resistant white paint (I used a pint size and it was plenty)
-sand paper 100 grit and 60 grit and maybe 40 up to you
-jitterbug (electric sander)
-L square

That's pretty cool, it looks very nice.