Overview
You can use Item Plan to generate Spreads (such as Size Spread or Channel Spread) that you can then use in Assortment Planning while quantifying your size level buys.
Steps
1 - On Item Plan go to the Toolio - Size Scale
view. This view allows you to filter your sales for the trailing 4 weeks for a particular style and groups the data by Size.
2- Select the sizes that you want to include in the Spread
3- Right Click > Generate Spread > Size
4 - Give a name for the the Size Spread and hit Generate
5 - Within Assortment Planning, while you're inputting your Size Spreads (or Size Scales), you will be able to select the Spread that you've created.
Video