Answered on github : https://github.com/ducksboard/gridster.js/issues/200#issuecomment-20469120
Found the problem, you have to set max_size_x and max_size_y
gridster=$(".gridster ul").gridster({ widget_margins: [6, 6], widget_base_dimensions: [59, 59], max_size_x: 9, max_size_y: 8 }).data('gridster');