.button {
background-color: #4CAF50;
border: none;
color: white;
width: 100%;
hight: 50%;
text-align: center;
text-decoration: none;
display: inline-block;
font-size: 16px;
margin: 4px 2px;
cursor: pointer;
}
table td form button{
width: 100%;
height: 100%;
}
<table width="100%" height="100%" border="1" cellspacing="0" cellpadding="0">
<tr><td><a href="proba.html" target="main" class="button">PROBA
</a></td></tr>
<tr><td><a href="about.html" target="main" class="button">ABOUT
</a></td></tr>
<tr><td><a href="courses.html" target="main" class="button">COURSES
</a></td></tr>
<tr><td><a href="department.html" target="main" class="button">DEPARTMENT
</a></td></tr>
<tr><td><a href="contact.html" target="main" class="button">CONTACT US
</a></td></tr>
<tr><td class="button"><a href="proba.html" target="main" >zxz
</a></td></tr>
<form action="proba.html" target="main" method="get">
<!-- First name: <input type="text" name="fname"><br> -->
<!-- Last name: <input type="text" name="lname"><br> -->
<!-- <button type="submit" formtarget="_blank">Submit to a new window</button> -->