I bike tour and use a Warbonnet Superfly. It is probably large enough to cover my hammock and bike, but I just leave my bike out in the rain.
Are you concerned rain will cause rusting or damage something? Do you ride in the rain?
I too was concerned about leaving my bike in the rain overnight and considered various options for covering it. However, after considering that fact that I routinely ride in the rain, it occurred to me that overnight rain won't likely hurt anything, and I carry a small bottle of oil for my chain in case it needs oiling. Other than a leather seat, or a seat or handlebar grips that absorb water, there probably is not much to worry about. This is not to say one should leave their bike outside to the elements for long periods of time, but for a touring trip there is likely little damage done.
Here's a bike forum post that provides different points of view on, and different options for, nightly coverage for bicycles while touring: