You can add height= and width= attributes to the IMG tag in your standard Reseller button code, like so:
<a href="http://www.e-junkie.com/?r=252475" target="ejcom" title="Shopping Cart by E-junkie"><img height="27" width="81" src="https://www.e-junkie.com/linkimg/5b5fa27b83b8ac696743aa2bcafdb5d4252475/1.gif" alt="E-junkie Shopping Cart and Digital Delivery" border="0"></a>
The above example provides the standard size, so multiply both figures by the same factor to enlarge the image without distorting it -- e.g., multiply both x2 to make the button twice as large. You could also just replace the src= URL with any custom image URL you wish, or replace the entire IMG tag with some custom link text, but note that would disable your ability to track Reseller link views in your Seller Admin > Reseller Account Details screen.