A corresponding cable is included in the scope of delivery, the device then converts the 12 V DC voltage into 230 V AC voltage. If technical support is required, I would contact the manufacturer.
The actual dimensions of Qoltec's 'Pure Sine Wave Inverter Monolith' vary from model to model, but for the specific model with ID 51942, no exact dimensions have been given in the available sources. However, for other similar models, dimensions have been reported as e.g. 105 mm x 245 mm x 55 mm.
If you are looking for the exact dimensions for the model with ID 51942, I recommend you consult the product manual or the manufacturer's official website for more precise information.