Heating Design tab on Model Options dialog.
The Heating Design margin is used to multiply calculated steady state heating loads in each zone to give a recommended heating plant size. It accounts for the additional heat required to bring the building up to temperature in a reasonably short preheat period and allows you to be confident that comfort conditions will be maintained in all but the most extreme winter conditions.
The default design margin is 1.25 which results in heating systems being oversized by 25%.