This paper targets the interdependence between sizing and power split optimization of hybrid energy storage systems (HESS) in electric vehicles (EV). In particular, a high energy (HE) density battery and an ultracapacitor (UC) hybrid system is investigated as a benchmark. A convex optimization problem is formulated to optimize the power split between battery and UC offline. Based on the power split optimization result, a HESS sizing optimization problem is developed to minimize the HESS weight and to fulfill the EV specifications of driving range and acceleration time.