column size and storage efficiency

My inclination when creating tables is to create columns using varchar-sizes in increments of 10 or 5 but I suspect
theremust be more-efficient sizes, specifically, values like 16, 32, 64 minus header-bits. 

- Is this true? Does it really have much impact on storage?
- How many bits are the headers?
- How does TOAST factor in?

Thanks,
Bruce Hyatt