We propose a two-parameter model for the perceived size (spatial extent) of a Gaussian-windowed, drifting sinusoidal luminance pattern (a Gabor patch) based on the simple assumption that perceived size is determined by detection threshold for the sinusoidal carrier. Psychophysical measures of perceived size vary with peak contrast, Gaussian standard deviation, and carrier spatial frequency in a manner predicted by the model. At suprathreshold peak contrasts Gabor perceived size is relatively unaffected by systemic noise but varies in a manner that is consistent with the influence of local contrast gain control. However, at and near threshold, systemic noise plays a major role in determining perceived size. The data and the model indicate that measures of contrast threshold using Gaussian-windowed stimuli (or any other nonflat contrast window) are determined not just by contrast response of the neurons activated by the stimulus but also by integration of that activation over a noisy, contrast-dependent extent of the stimulus in space and time. Thus, when we wish to measure precisely the influence of spatial and temporal integration on threshold, we cannot do so by combining contrast threshold measures with Gaussian-windowed stimuli.