OP could definitely have used more examples, but I think he's on the right track. Where declarative or functional programming comes in really handy is composition. Underscore has a lot of utilities that make it easy.

  var genericFilter = function(type, value) {
    return function(items) {
      return _.filter(items, function(i) {
        return i[type] === value;
      });
    }
  };

  var sizeFilter = genericFilter('size', selectedSize);
  var brandFilter = genericFilter('brand', selectedBrand);

  var appliedFilters = _.compose(sizeFilter, brandFilter);

  var filteredItems = appliedFilters(items);
  // which ends up doing  sizeFilter(brandFilter(items));

// edit for sloppy code;