Id have to say the reptile community is more like the saltwater fish community. There are some things that most people agree on, but there are heated debates on many things, this being one of them.
It really varies greatly by species, you need an enclosure large enough for the animal to do what it needs to do, thermoregulate for example, (though even this is debated nowadays.)
So let's take three different species, Red tailed boa, a carpet python, and a green tree python. And assume we are talking about a snake of each between 5-6 feet.
Boa constrictors are primarily terrestrial, so they need ground space. You can utilize some height and have ledges, etc. My personal rule of thumb is to have an enclosure that the length + the width is at least the length of the snake. So a 6" Boa would need an enclosure that is 4'X2'x1'. Even if you had more height, I wouldn't reduce the floor space by much.
A GTP of the same size would need some height, at the expense of the floor space if necessary. 3'x2'x2' would work well. Even 3'x1.5'x2'. Of course, for a well established adult animal, bigger is almost always better.
A carpet python of the same length, being semi arboreal, will use both floor space and height equally. So you have more flexibility.
There are other things that need to be considered here, what type of heat source you are using, humidity, etc.
Lizards are completely different in many ways. Especially if it is a diurnal basking species requiring UVB, i.e Agamids and Iguanids. They are typically more active, and many species require a hot basking spot (similar to the heat you would find in direct sun in the tropics or in a desert setting.) So if you have a 100°-120° you also need enough space for it to have a cooler area.
