When changing CSS like you ask, use your browser's inspection tool to find the CSS impacting the field or container. You can then adjust the CSS to your liking by overriding Formidable's defaults. In this case, add this you your custom CSS:

.frm_html_container.frm_scroll_box,
.frm_form_field.frm_html_scroll_box {
height: 200px !important;
}