You can get a smaller hood for the bulb, but the smallest size that the UVB strip fluorescent comes in is 18". Here's a picture of a strip light to show you what beardie parents was talking about:
http://www.petmountain.com/product/light-fixtures/516021/perfecto-fluorescent-perfect-a-strip-light-black.html
Does that help?
http://www.petmountain.com/product/light-fixtures/516021/perfecto-fluorescent-perfect-a-strip-light-black.html
Does that help?