Basically in your themes CSS it’s specifying the width of the “input” field… I don’t know why but some themes do this. I prefer to leave it so the width can be the width of the image.
Anyway, Here are a few options for you:
Option 1:
If you look at the CSS file of your theme you will see the following bit of code:
#page_left input,
#page_left select,
#page_left textarea
{
border:0px;
padding:6px;
width:550px;
background: transparent url(../../images/10/sub_back.png) top center repeat-x;
border:1px solid #ededed;
font-family: ‘Lucida Grande’, ‘Lucida Sans Unicode’, Tahoma, Arial, san-serif;
}
Get rid of this line that is specifying the width of the button and it should work:
width:550px;
Option 2:
The other option is to introduce the following CSS in the “wp_eStore_style.css” file (this file is located inside the WP eStore plugin directory):
.eStore_button
{
width:84px;
height:50px;
}you will have to adjust the width and height according to the image you are using for the “Add to Cart” button.
Option 3:
Get a theme that doesn’t have crappy CSS 
(just kidding)