If you want to change the size of the image that appears on the single product details page, you will be working with this CSS code:

#view .sp .rc
{
	margin-left: 210px;
	padding-left: 10px;
}
#view .sp .lc .thmb { width: 200px; }
#view .sp .lc .thmb img
{
	max-width: 200px;
	width: expression(this.height < this.width && this.width > 200px ? '200px' :true);
}

Let's say you want to increase the size of the image by 85px. Here's what the edited code would look like:

#view .sp .rc
{
	margin-left: 295px;
	padding-left: 10px;
}
#view .sp .lc .thmb { width: 285px; }
#view .sp .lc .thmb img
{
	max-width: 285px;
	width: expression(this.height < this.width && this.width > 285px ? '285px' :true);
}

Paste the updated code into the Custom CSS field for your store (Factory > Store menu > Custom CSS) then update your store.