Do not try to resize the celltable, resize the div (Flowpanel or similar) that is holding the celltable.
>> By default say I raw a cell table with 25 rows and when the the window is shrink ed in size to show 20 rows , then scroll bar should automatically appear on the table .
The browser must automatically handle this for you.
Make sure you have the css property,
div.celltable-container {
overflow: scroll;
}