In browse mode, there is no such capability. In preview/print or when saving as PDF, you can set the portal to "slide up" , "resize enclosing part" on the inspector's position tab. This will only shrink the portal to fit, it will not expand it to fit.
But if you are doing this in order to print your quote, there's a much better alternative:
Base a layout on the line items table instead of its parent table. Make this a list view layout and use a script from your current layout that either performs a find or uses Go To Related Record to pull up the line items in a found set. You can include fields from the parent table in the header, footer or a grand summary layout part to complete your "quote". The list view layout will then be a layout that automatically adjusts to suit any number of line items. The invoice starter solution that comes with your copy of FileMaker uses this method to print an invoice is you want to examine an example of this.
Great, thank you for the help