At least in Safari and Konqueror, multiple select list are not rendered correctly, if size attribute is not set.
If there is no size attribute, which tells the renderer the number of rows to show in a multiple select list, there will only be one row at least in Safari and in Konqueror.
This is a usability problem. Try to select multiple values, if there is only one row.
The number of rows shown in a multiple select list should be set to a default value (like size="5") or, even better, make it configurable.