What is the maximum height of a plant that I can put inside.
137 ft. 3½ in.
exactly.
But seriously, it depends on a few factors. Some plants could be 2 ft. when you put them in, and grow to 4' within a few weeks. Others could be 40 in. and take months to grow another inch. This is why it's somewhat important to know the growth rate (fast or slow) of the plants you choose, as well as lighting & humidity requirements (most cacti don't do well in chameleon enclosures

)
Lighting, IMO, can go either way, (across or diagonal) depending on how you set things up. I prefer to mount basking lights at an angle (as opposed to straight down) to produce an ellipse rather than a circle. This allows a basking gradient; closer to the bulb, the temp is warmer, while further away it is cooler. Lizards
generally know what they want, and can have different moods on different days.
Here's a recent post on basking lights.
I agree with setting up the basking area first (height, distance from both lights, temperatures) and filling in plants later. These will consist of both center piece plants and "fill-ins" to provide both partial and complete areas for a cham to hide—both from lighting and from view (some can be quite shy, and all are different.)

Of course at least some of the basking area should be in the open for the times when your cham wants it that way (usually for morning warm-up).
Choose plants (IMO) that can tolerate some pruning in case they do grow too quickly or too tall.