The Beetle Bags openings 6.5 inches wide by aprox 12inches in length and somewhat triangular in shape. I took a 7 day trip and packed everything I needed in them.
I'm a true beliver in the more room I have for packing the more I will take, so I just learned to take less items, jeans, socks, t-shirts, electric vest, some tools is about all I take on trips now.
No way can you fit a helmet.