A very rough concept for a collection that fetches pages of data with varying-length lists .

VariablePageSizeCollection.js

var RandomSize = Backbone.Collection.extend({
    pageSize: 25,
    nextPage: 1,
    fullCollection: undefined,
    url: '/forensics/api/random/',
    parse: function(resp) {
        return resp.data
    },
    getNextPage: function() {
        var self = this;
        var currentLength = this.fullCollection.length;

        if (currentLength > (this.pageSize*(this.nextPage-1) + this.pageSize)) {
            _fillNextPage();
        } else {
            this.fullCollection.fetch({
                kache:false,
                remove:false,
                url: this.url,
                success: function() {
                    if ((self.fullCollection.length - currentLength) < 25) {
                        self.getNextPage();
                    } else {
                        _fillNextPage();
                    }
                }
            });            
        }

        function _fillNextPage() {
            var start = self.pageSize*(self.nextPage-1);
            self.add(self.fullCollection.slice(start, start+self.pageSize));
            self.nextPage += 1;            
        }
        return this;
    }
});

var rs = new RandomSize();
rs.fullCollection = new RandomSize();