You want the portal record rows to change in number with the number of records shown by it? So if one is shown, then there would be one portal row, and if five then five et? ... Don't think that can be done, unless you want to have as many layouts as the could be portal records...
Thank you for your post.
The answer by "obeechi" is correct (Thank you!).
When you define a portal on a layout, you specify the number of rows you want displayed. FileMaker Pro does not have the ability to reduce the size of a portal depending on the number of records. The same would be true for any field. For example, if a text field sometimes two lines, you cannot have the text field automatically expand to two lines.
For printing purposes, you can use Format | Set Sliding Printing... to make your portal "shrink" when it is previewed or printed. That may accomplish what you need. Just keep in mind that this feature is not visible in browse mode.