Sketching allows designers to quickly explore various product ideas while still in the early stages of the design process. However, the scale and proportion of a final product can be difficult to imagine when sketching in a 2D medium such as pen and paper. On-tablet 3D sketching (using a pen and a graphics tablet) allows designers to create 3D shapes with 2D pen strokes and traditional 2D perspective drawing skills. However, interactions with the 2D screen can hinder designers from envisioning a realistic scale and proportion for their products. On the other hand, in-space 3D interactions (using hand-tracking and virtual reality technologies) can allow designers to create 3D shapes in the air using their hands and their inherent spatial senses. This enables designers to express and evaluate products’ scale and proportion using an immersive stereoscopic view; however, low precision and fatigue mean that this technique is impractical in product design. In this study, I propose a new design workflow that combines on-tablet 3D sketching and in-space 3D interactions in a complementary manner. In this workflow, a designer quickly expresses an approximate scale and proportion for a product using hand posture and hand motions captured with a hand-tracking sensor. The designer can then use the expressed information as a 3D reference for a detailed pen drawing on a graphics tablet, thus creating a 3D product concept. The designer can immersively experience and evaluate the created concept in virtual reality through a head-mounted display. The designer can perform all these activities in an integrated workflow to iteratively and progressively develop a product concept while maintaining its scale and proportion. I designed interaction techniques to effectively sketch products of various sizes with a guaranteed scale and proportion and integrated those techniques into a single workflow. I then implemented the workflow as a practical system using off-the-shelf devices. I evaluated the system with end users and analyzed the design results and processes. Finally, I applied the system to actual product design processes to show that the new 3D sketching workflow has the potential to be an innovative design process in the real world.