I know of the OTF "optical sizes" feature, but I don't know how to use it properly. How can I check which optical sizes are available for my font?
E.g. I have Adobe Caslon Pro which has the aforementioned feture enabled. I browsed the TEX.SE website, but all I found is related to Minion Pro which has different files for different sizes (as I understand). My Caslon does not. So how can I exploit the feature?
Please mind that my main concern regards LuaLaTeX, which I'm using, but for completeness I think that having the XeLaTeX way in the answer too wouldn't be bad (esp. if they differ).