We investigated exogenous effects of stimulus size and duration on the visual alpha oscillatory activity in human EEG. EEG was collected from 17 subjects while they performed a simple choice reaction task requiring discrimination of squares and circles. In separate blocks stimulus size and duration was manipulated. EEG was recorded from 64 electrodes. A wavelet transform based on Morlet wavelets was employed for the analysis of individual alpha activity. Size modulatory ERP deflections showed significant increases in the amplitude of P100 for enhanced stimulus size. No such effect was observed for the amplitude of the N170. The evoked alpha showed onset and offset responses, whereas induced activity showed only a single decrease in response to stimulation. As the size of presented stimulus increases, evoked alpha response becomes larger in its amplitude for stimulus onset and offset. In addition, there is a modulation of individual evoked alpha activity by stimulus-shape for onset responses in posterior regions. According to these data, evoked and induced alpha activity show different reaction patterns to exogenous stimulus properties. It probably implies that these two kinds of alpha activity have different underlying mechanisms, which may serve different cognitive functions. Together with results from literature, this exogenous modulations of alpha activity demonstrate that the human alpha rhythm does reflect top-down as well as bottom-up processes.