That's not something you can do in FileMaker. The number of portals are fixed. You can set the portal to have a very large number of portal rows and set it to "Slide up, Resize enclosing part" and then the unused space in the portal will be removed when you print or preview the report.
Another option that is possible is to not use a portal at all but use a list view layout based on the portal's table. Data from the parent table can be put on this layout--often in headers and/or sub summary parts. Such a layout is usually used for printing, but it can be used for data entry also.
Hey, but i saw somewhere, such a problem has a solution, may be.. making use of some external plug-in....
pls reply for any idea.
Best as I can tell you cannot change the size of a portal by itself. However, if you anchor the portal to the bottom of the window you can change the size of the window based on the found count of the portal, a script trigger and resize window script step, within a nested if or case statement.
I'm not sure how much your users will appreciate this "benefit" over the window resizing vs. using the scroll bar, but it will increase the number of visible rows.