You can't replace the page drop-down but you can append an input box to the pagination bar, please refer to the code below:
var pager = $('#dg').datagrid('getPager');
var opts = pager.pagination('options');
var list = pager.find('.pagination-page-list').hide();
var tb = $('<input>').insertAfter(list);
tb.numberbox({
width: 50,
value: opts.pageSize,
onChange: function(value){
list.html('<option>'+value+'<option>');
pager.pagination('refresh', {pageSize:value});
list.triggerHandler('change');
}
})