How to change size of nested form?

Hi, I would like to add a Nested Form within a Form. So I added a new field of type Nested Form and added my Data Source Item. The linked Item is shown correct, but the default size of the nested box is to small. How can change increase the size? I have already tried using the following code in the Field CSS tab:
{$this field rule} .sys_ft_nested_form { width: 900px; height: 700px; } {$this field rule} { margin: 5px 0px 5px 0px; width: 900px; height: 700px; }
Unfortunately, that was not the solution. Any ideas? Thanks again!