Personally I think the ideal size is Medium. According to the 2024 D&D Player's Handbook: "A Medium creature controls a 5 by 5-foot square on a grid (1 square). This is the exact same space controlled by a Small creature". In the 2014 ruleset, Small creatures were at disadvantage when wielding heavy two handed weapons, but now it's tied to strength requirements. Generally, though, Medium creatures are still ideal for most melee combats due to being able to grapple creatures one size larger.