Button size and format is dependant on the styling your website theme uses. So you can add a class to it, like... 'contact-submit' and then in the stylesheet of your theme add few lines with the styling to it, for example in my case lately I've added:
.contact-submit {
background-color: #000000 !important;
border-radius: 0 !important;
margin-top: 10px;
padding: 10px 75px 10px 74px !important;
text-shadow: 0 1px 1px rgba(0, 0, 0, 0.3);
}
That overwrote styling that my theme was using, and made button look as I want it to look.
P.S.
I'm not the expert yet! Learning stuff as well, it's just what worked for me.