Using a small-angle X-ray scattering technique, shape and size, and internal structure of diglycerol polyisostearate nonionic surfactant micelles in nonpolar oil n-hexadecane (HD) were investigated at 25 degrees C. Furthermore, the effect of added water on the structure of host reverse micelles was also investigated. The scattering data were evaluated by the generalized indirect Fourier transformation (GIFT) method and model fittings. It was found that diglycerol polyisostearate (abbreviated as (iso-C18)nG2, where n=2-4 represent the number of isostearate chain per surfactant molecule) spontaneously form reverse micelles in HD at 25 degrees C and their geometry (shape and size, and internal structure) could flexibly be controlled by a small change in the lipophilic tail architecture of the surfactant, temperature, and water addition. Increasing number of isostearate chain per surfactant molecule decreases the micelles size favoring prolate-to-sphere type transition. This phenomenon could be best understood due to voluminous lipophilic part of the surfactant. Increasing temperature decreases the size of the reverse micelles due to enhanced inter-penetration of the surfactant chain and the oil and also due to dominant hydrophobic character of the surfactant at higher temperatures. In the studies of effect of added water on the structure of micelles, it was found that the reverse micelles swell with water causing two dimensional micellar growths.