Abstract: Deepwater subsea wellhead systems supported by suction piles are characterized by large diameters and shallow embedment depths. However, existing design methods are mainly derived from suction caisson foundations used in offshore wind turbines, and a systematic structural parameter optimization framework tailored to the loading conditions and functional requirements of subsea wellheads is still lacking. To optimize the combination of suction pile diameter and penetration depth while satisfying the requirements for wellhead stability, alignment accuracy, and verticality control, theoretical analysis combined with laboratory model tests was adopted to investigate the penetration and bearing performance of suction piles with different configurations. Suction pile models with diameters ranging from 10 to 50 cm and corresponding length-to-diameter ratios of 1.2–6.0 were designed under the condition of equal lateral surface area. Gravity penetration, suction-assisted penetration, and vertical and lateral bearing capacity tests were conducted under different suction flow rates and seabed soil undrained shear strengths (20 and 25 kPa), and the sensitivity of various parameters to installation performance and bearing capacity was analyzed. The results show that increasing the model diameter from 10 cm to 50 cm reduces the gravity penetration depth by 58%–62%. Increasing the suction flow rate from 20 L/min to 60 L/min increases the ultimate penetration depth by an average of 35%–45%. Under equal lateral surface area conditions, the difference in vertical bearing capacity among suction pile models with different dimensions is less than 5%, while the lateral bearing capacity increases by 48.7% with increasing diameter. Based on the experimental results, a design chart for selecting suction pile dimensions under equivalent vertical bearing capacity conditions was developed, and a structural selection framework satisfying the design requirements was established. The findings provide theoretical support and design references for the structural optimization and field installation of deepwater suction-pile-supported subsea wellhead systems.