Resonance and frequency-domain effects are well documented for floating bodies such as barges and moored ships. If a body’s waterplane dimensions are small in comparison to the wavelength, it can be treated as a point absorber, and its response can be found in terms of response amplitude operators (RAO’s) which are functions of the wave frequency. However, for wave-energy devices and breakwaters, the waterplane dimensions of the body are often of the same order as the wavelength, so wavelength effects become important. Simple relationships between the wavelength and the response in different degrees of freedom would be very useful in the design of buoyant ocean wave-energy conversion devices. In the present study, a numerical integration scheme is used to solve an initial-value problem and simulate the motions of a rectangular prismatic floating body in regular waves. Wave forces are calculated numerically at each time step using results from a panel-based potential flow method. While a linear frequency domain solution would be easier, the present method is employed to allow for nonlinear analysis in the future. Response amplitudes in heave, pitch, and surge are plotted versus wave frequency (normalized by the barge undamped natural frequency in the appropriate degree of freedom) and simultaneously versus wavelength (normalized by the barge length). This amounts to an extension of the traditional two-dimensional RAO (response versus frequency) to a three-dimensional plot (response versus frequency and wavelength). The simulated data agree well with expectations, and they show that the wavelength-to-body-length ratio is an important factor in the design of floating bodies, especially buoyant ocean wave-energy conversion devices.