Electrochromic devices (ECDs) play an important role in smart window applications for blocking heat from sunlight. Tungsten trioxide (WO3) is one of the best candidates for making electrochromic films. Current manufacturing processes are, however, costly, have long processing time, and often use non-eco-friendly precursors. Here an alternative facile method integrating electro-exploding wire and spray coating techniques is demonstrated for applications in large size ECDs. The electro-exploding wire technique is used to synthesize WO3 nanomaterials (NMs) in deionized water. The varied size distribution of WO3 NMs is achieved by changing the exploding voltage. The NMs are characterized by X-ray diffractometer (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The exploding voltage of 36 V is selected to generate WO3 NMs in a wide size distribution for making porous WO3 films. Subsequently, spray coating is used to disperse WO3 NM suspension on the ITO/glass substrate to form an electrochromic film. The films are characterized by scanning electron microscopy (SEM) and by cycle voltammetry, chronoamperometry, and chronocoulometry measurements in dilute sulfuric acid. The WO3 NM films prepared at a high exploding voltage and a high substrate temperature present a higher electrochemical stability. Considering porosity and electrochemical stability, WO3 NMs exploded at 36 V are used to prepare WO3 films on substrates heated at 300 °C. The electrochromic WO3 film can be operated for over 1000 cycles. We demonstrate an ECD with an area of 15 × 15 cm2 and propose a way to make large size ECDs with low cost and eco-friendly processes.