As a race director, you can set yourself up for successful ordering by asking participants to specify their shirt size during online race registration.
Registration should also include a clearly posted deadline that runners must meet to be guaranteed a shirt. The deadline you set should be around 3-weeks before your event date. For covering stragglers, you can add on 15-25% to your final order.
In order to allow time for artwork adjustments and last-minute sponsor additions, it is recommended that you start your apparel order 4 to 6 weeks prior to your event date.