That's easy!
First, give your button an id such as <input type="button" value="I am a button!" id="myButton" />
Then, for height, and width, use CSS code:
.myButton {
width: 100px;
height: 100px;
}
You could also do this CSS:
input[type="button"] {
width: 100px;
height: 100px;
}
But that would affect all the buttons on the Page.
Working example - JSfiddle