arXiv:2611.09001v1 Announce Type: new Abstract: Interconnecting large data centers can be delayed by grid requirements on demand flexibility, ramping limits, and ride-through performance. This paper proposes an optimization-based framework to size a Battery Energy Storage System (BESS) that accelerates data center grid integration by providing BESS capacity required to meet selected flexibility and grid-support requirements. The model co-optimizes BESS power rating and energy capacity while allocating capability across three services: (i) demand response over a specified duration, (ii) mitigation of high-magnitude, high-frequency load fluctuations through reserved active power with zero net state-of-charge change, and (iii) voltage support via reactive power capability at the point of common coupling. Minute-level operational scheduling enforces state-of-charge feasibility and load ramp constraints, and incorporates a controllable queue of delay-insensitive computing tasks to enhance flexibility. A case study using a real data center load profile demonstrates the proposed method and quantifies the BESS power-energy requirements and operational trade-offs needed to satisfy the multi-service constraints, providing practical guidance for interconnection planning and deployment.