You are basically looking for the API section. Having extensively used this plugin myself, I know exactly what you're looking for:
var api;
var cropWidth = 100;
var cropHeight = 100;
$(window).load(function() {
// set default options
var opt = {};
// if height and width must be exact, dont allow resizing
opt.allowResize = false;
opt.allowSelect = false;
// initialize jcrop
api = $.Jcrop('#objectId', opt);
// set the selection area [left, top, width, height]
api.animateTo([0,0,cropWidth,cropHeight]);
// you can also set selection area without the fancy animation
api.setSelect([0,0,cropWidth,cropHeight]);
});
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