In Stock Days and Just in Time strategies you will set the days of stock you want to hold and the size of the replenishment will be determined based on how much demand occurs in the following days. If you need to set a replenishment size, you can change the strategy to Manual and set the min / max stock control to differ by your replenishment size. Whenever stock on hand reaches the min, it will replenish up to max. 
Otherwise, you can simply take the recommendations from AIM and enter replenishment transactions that fit your minimum size. These replenishments will then be loaded into AIM and the forecast will incorporate the future stock movements.