This paper empirically investigates how board characteristics—gender diversity, ethnic diversity, foreign participation, and boardindependence—affect firm performance (ROA, ROE, and Tobin's Q) among the top 100 FTSE Bursa Malaysia non-financialfirms. Further, we examine the moderating role of firm size on the relationships between board characteristics and firm per-formance. The dataset comprises 800 firm-year observations for 2015–2022. Our study uses interactive variables to assess theeffect of gender diversity, ethnic diversity, and foreign participation on firm performance. The findings reveal that firm sizealters the impact of board characteristics on firm performance. Specifically, gender diversity's influence shifts from negative topositive with larger firms, while ethnic diversity and board independence exhibit the opposite trend when measured by Tobin's Q.Additionally, firm size weakens the positive effect of foreign participation on ROE, rendering it insignificant. The study also findsthat ethnic diversity within gender-diverse boards negatively but significantly impacts firm performance. This study challengesthe uniform assumptions about board diversity's impact on firm performance, demonstrating context-dependent variations. Itextends corporate governance literature by revealing firm size as a critical moderator in board-performance relationships andhighlights inconsistencies in board diversity effects within an emerging economy context. The findings suggest policymak-ers and corporate leaders should tailor board diversity strategies based on firm size to maximize performance benefits. Largerfirms may leverage gender-diverse boards for positive outcomes, while ethnic diversity and board independence require carefulmanagement to avoid unintended negative effects. Regulators should consider firm size when designing corporate governancepolicies to enhance firm performance in emerging markets