<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
</head>
<body>
<input type="text" value="TR150082900009492038044184" style="width: 300px; " id="kopyala">
<button onclick="myFunction()">İBAN NUMARASINI KOPYALA</button>
<script>
function myFunction() {
var copyText = document.getElementById("kopyala");
copyText.select();
copyText.setSelectionRange(0, 99999)
document.execCommand("copy");
}
</script>
</body>
</html>style içindeki width: 300px; 300 yaptım kendine göre ayarla veya direkt width: 100%; tüm ekran dollarsa 70%/80%'e düşür.