how to make the send button bigger?

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.