If you have any choice in the timing, schedule your lecture for the late morning, which is the optimum time for audience concentration. First thing in the morning, much of the audience will not yet have woken up, after lunch they suffer post-prandial stupor and in the late afternoon they tend to run out of steam. In addition, if you have a say on location, select a venue that the audience will comfortably fill. Using a 200-seat lecture theatre for an audience of 10 will lead to loss of intimacy and using a room that holds 20 for a lecture to 100 will lead to frustration from overcrowding