Sometimes you find it very frustrating to set the lightning-textarea component’s height. This task is the most simple one with HTML Textarea; you just set height attribute. However, with lightning-textarea, there is no height attribute; the custom CSS file doesn’t work. How we do it?
It may simple than you thought, we will create a style tag and add it into lightning-textarea.
<template>
<lightning-textarea value={inputValue}></lightning-textarea>
</template>
import { LightningElement, track } from 'lwc';
export default class LightningTextarea extends LightningElement {
isrendered = false;
@track inputValue = "Set Height Sample";
renderedCallback(){
if(!this.isrendered){
this.isrendered = true;
const styleTag = document.createElement('style');
styleTag.innerText = "textarea { min-height: 250px; }";
this.template.querySelector('lightning-textarea').appendChild(styleTag);
}
}
}

Ways easier. Just put this into the CSS file:
:host {
–sds-c-textarea-sizing-min-height: 250px;
}
Thanks, Mate, but this Styling Hooks feature is on Beta. Probably, We’ll still switch to new styling hooks in near future.