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This thesis investigates how Discrete Event Simulation (DES) can support internal logistics design and fleet size optimization in a new manufacturing facility during the concept design phase. The study addresses the challenge of dimensioning internal transport resources, such as forklifts and AMRs, when operational data are limited and the facility exists only as a CADbased layout. A mixed-method case study was conducted in collaboration with Volvo Construction Equipment. Interview data from forklift operators and a manager were used to estimate operational parameters such as loading and unloading times, vehicle speed, charging behaviour, task volume, and congestion effects. These inputs were combined with CAD-based process-time calculations and implemented in a DES model developed in FACTS Analyzer. Two alternative layout variants were evaluated: a Main Layout and a Footprint Reduction Layout. Scenario experiments were performed for different forklift and AMR fleet configurations, and the results were assessed using throughput, utilization, tasks completed per day, and bottleneck behaviour. The results show that increasing fleet size mainly reduces resource utilization, while annual throughput changes only marginally. For forklifts, the smallest tested configuration, 1 inbound forklift and 1 supermarket-area forklift, was sufficient to meet the modelled production target. For AMRs, one vehicle was sufficient under the tested assumptions. The simulation also showed that the Footprint Reduction Layout produced slightly different transport cycle times, but the effect on overall fleet requirements was limited. Bottleneck analysis indicated shifting constraints rather than one stable system bottleneck. The thesis concludes that DES is a useful decision-support tool for early-stage internal logistics planning in greenfield manufacturing environments. It helps identify minimum viable fleet configurations and shows that layout changes may affect local transport performance without necessarily changing overall system output.