If you want to call a different size for the button inline, you would probably do it like this:
<div class="button" style="width:60px;height:100px;">This is a button</div>
Or, a better way to have different sizes (say there will be 3 standard sizes for the button) would be to have classes just for size.
For example, you would call your button like this:
<div class="button small">This is a button</div>
And in your CSS
.button.small { width: 60px; height: 100px; }
and just create classes for each size you wish to have. That way you still have the perks of using a stylesheet in case say, you want to change the size of all the small buttons at once.