Size charts for what? Wall thickness? A particular manufacturer may have "standard" vessels that could be cheaper than a custom one if you don't need any special connections or other features.
In my experience you determine the vessel size, head type, nozzle locations, etc and a manufacturer will determine the wall thickness based on material, temperature and pressure. They'll usually include wall thickness in their initial quote.
Also, if you meant 15 psig in the original post, note that is NOT atmospheric. You'll confuse both tank and vessel manufacturers if you're saying 15 psig is atmospheric.