Vertical and horizontal spaces between products can be easily changed by adding some CSS styles in your CSS theme:

In order to change space between products you should use following code:

td.ecwid-productBrowser-productsGrid-cell {
   padding: 0px 2px; /*change 2px value to make space between products bigger or smaller*/
}

In order to make spaces between products lines smaller you should add this code in your CSS theme:

td.ecwid-productBrowser-productsGrid-cellSpace { 
    height: 5px;
}