Using CSS you can set a style for that specific button using the id (#) selector:
#search {
width: 20em; height: 2em;
}
or if you want all submit buttons to be a particular size:
input[type=submit] {
width: 20em; height: 2em;
}
or if you want certain classes of button to be a particular style you can use CSS classes:
<input type="submit" id="search" value="Search" class="search" />
and
input.search {
width: 20em; height: 2em;
}
I use ems as the measurement unit because they tend to scale better.