if you use Form, i found this in the javadoc:
Layout
Layout policy in Form is organized around rows. Rows are typically related to the width of
the screen, respective of margins, scroll bars, and such. All rows in a particular Form will
have the same width. Rows do not vary in width based on the Items contained within the
Form, although they may all change width in certain circumstances, such as when a scroll
bar needs to be added or removed. Forms generally do not scroll horizontally.
Forms grow vertically and scroll vertically as necessary. The height of a Form varies
depending upon the number of rows and the height of each row. The height of each row is
determined by the items that are positioned on that row. Rows need not all have the same
height. Implementations may also vary row heights to provide proper padding or vertical
alignment of with Item labels.
...
extract from Form javadoc
http://java.sun.com/javame/reference/apis/jsr118/javax/microedition/lcdui/Form.html