The height is set to @baseLineHeight * 3 + 8px (see the reference to the source below). To override this, you can add something like .orbeon .fr-view textarea { heigth: 100px } in say custom.css, that you place under WEB-INF/resources/config, and reference from a property <property as="xs:string" name="oxf.fr.css.custom.uri.*.*" value="/config/custom.css"/> in your properties-local.xml.
https://github.com/orbeon/orbeon-forms/blob/4.5-ce/src/resources/apps/fr/style/form-runner-bootstrap-override.less#L16Alex
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