How can I increase the size of a text field with multiple lines of text on the page?
In the classic Oracle Sales, you can't control the size of a long text field on the page. The size is set dynamically. In Oracle Sales in the Redwood User Experience, you can use Oracle Visual Builder Studio to create field templates to control the size of text fields on the page.
See the topic Control How a Field is Rendered with Field Templates in the Extending Oracle Cloud Applications with Visual Builder Studio guide.