The demand for environmentally friendly aviation has increased the interest in alternative concepts, such as hybrid-electric propulsion systems. A novel conceptual hybrid-electric sizing methodology was devised, which provides a detailed engine performance prediction and an accurate sizing of the powertrain components. The performance prediction methodology individually scales user-provided origin fan and engine core performance maps to generate a coherent engine performance map according to prescribed point performance requirements, while the component sizing methodology estimates individual component masses and determines their positions based on defined knowledge-rules. For each methodology, a dedicated design tool was created for easy integration into conceptual design workflows. A verification of the methodology via case studies assessing generated example aircraft concepts confirmed not only the validity of the obtained results, but also demonstrated a high accuracy at low computational costs. The exhibited robustness and sensitivity encourage the use of the methodology for future studies on hybrid-electric propulsion.