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.

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.