Test if size has a unit, otherwise appends the passed unit string, or the default for this Element.
@param size {Object} The size to set
@param units {String} The units to append to a numeric size value
@private
@static
function(size, units) {
            // Most common case first: Size is set to a number
            if (typeof size == 'number') {
                return size + (units || this.defaultUnit || 'px');
            }

            // Size set to a value which means "auto"
            if (size === "" || size == "auto" || size === undefined || size === null) {
                return size || '';
            }

            // Otherwise, warn if it's not a valid CSS measurement
            if (!this.unitRe.test(size)) {
                //<debug>
                if (Ext.isDefined(Ext.global.console)) {
                    Ext.global.console.warn("Warning, size detected as NaN on Element.addUnits.");
                }
                //</debug>
                return size || '';
            }

            return size;
        }