// initialize gridster with 200x200px widgets using a 5px margin
var gridster = $('.gridster ul').gridster({
    widget_base_dimensions: [200, 200],
    widget_margins: [5, 5]
}).data('gridster');

// resize the widgets to 300x300px and 10px margin
gridster.resize_widget_dimensions({
    widget_base_dimensions: [300, 300],
    widget_margins: [10, 10]
});