Setting the C1ListViewInputItem.Type property to 'textarea' gives you a resizable text input tool. This input tool will automatically resize itself as you enter text, or you can resize it yourself using the resize handle.

You can set the Type property using markup similar to the following:

<cc1:C1ListViewInputItem ID="SubjectInput" LabelText="Subject" Type="textarea"></cc1:C1ListViewInputItem>