The fit addon provides the fit method that lets you adjust the size and geometry (columns 𝗑 rows) of the terminal to fit the size of the parent element.
import { Terminal } from '@xterm/xterm';
import { FitAddon } from '@xterm/addon-fit';
const term = new Terminal();
const fitAddon = new FitAddon();
term.loadAddon(fitAddon);
// Open the terminal in #terminal-container
term.open(document.getElementById('terminal-container'));
// Make the terminal's size and geometry fit the size of #terminal-container
fitAddon.fit();