To style the input field of a form, you can try to run the following code. Style the <input>:
Example
<!DOCTYPE html>
<html>
<head>
<style>
input {
width: 100%;
}
</style>
</head>
<body>
<p>Fill the below form,</p>
<form>
<label for = "subject">Subject</label>
<input type = "text" id = "subject" name = "sub">
<label for = "student">Student</label>
<input type = "text" id = "student" name = "stu">
</form>
</body>
</html>