Change default input parameter drop down list size

I am working on a report which uses a multi value input parameter. The value
set is depend on the other input parameter, so the size and value are
different when the value of the other parameter has changed. The problem is
sometime it works perfectly, but the other time when there are only two
value, the drop down list will show "Select All" and first item and gives me
a scroll bar. This is very annoying. Anyone know how to make the drop down
list to disply more desirous number of items. In addition, is there a way to
control the parameter text box length?I have the same problem but even a worse scenario. My list is filtered by
another and can end up with only one entry. While two entries at least lets
you scroll through the list ('cause the scroll buttons are shown), a list of
only one value doesn't even show the scroll buttons.
I also need a way to allow/force a list this short to be at least long
enough to show the scrollbars.
Jon