Experimental physical model studies of hydraulic structures are often conducted to replicate flow behavior that may occur at the prototype scale. Geometric similitude is most often maintained between the prototype and model when studying reservoir and open channel hydraulic structures to account for the dominant gravity and inertia forces while other fluid forces (e.g., viscosity,surface tension) are assumed negligible. However, as model size and/or upstream head decreases, other fluid forces can exceed the negligible level and influence model flow behavior. This phenomenon is referred to as size-scale effects and is one potential origin of error in predicting the prototype behavior through testing geometrically similar models. To extend the existing research of size-scale effects on nonlinear weirs half-and quarter-round trapezoidal labyrinth weirs and piano key weirs were fabricated at length ratios of 1, 2, 3, 6, and 12. The largest weir model for each weir type (i.e., a weir height of 36 in for labyrinth weir models and a weir height of 33 in for piano key weir models) served as the corresponding prototype.Weir models were hydraulically tested to assess differences among head-discharge relationships and flow behavior. Limiting criteria were recommended to avoid size-scale effects depending on the weir type and model size. The results of this study will help hydraulic modelers determine what limiting criteria should be met to avoid size-scale effects.