Edit:
@t1m0thy's answer is more elegant than mine, better follow his advices.
Also, nice link proposed by @aldemarcalazans in the comments: https://davidwalsh.name/html5-buttons.
Original answer:
Use when you need a link (the a of anchor). Use
when you need a button.
That said, if you really need to expand an , add the CSS attribute
display: block;
on it. You'll then be able to specify a width and/or a height (i.e. as if it were a ).