Short note about using XeLaTeX and LuaLatex with EB Garamond via fontspec.

EB Garamond fonts are loaded through fontspec with

\setmainfont{EB Garamond}

This will load the appropriate fonts for the actual sizes, currently EBGaramond08* for sizes up to 10pt and EBGaramond12* from 10.1pt onwards.

If you want to stay with one design size, you have to load its regular font and switch off optical sizes:

\setmainfont[OpticalSize=0]{EB Garamond 12 Regular}

If you wish to use a specific font file, e.g. EB Garamond Italic:

\setmainfont{EB Garamond 12 Italic}

N.B. It is preferable to use the +c2sc and +scmp features for Small Caps, rather than the SC or ALLSC font files. 

\newfontfamily{\smallcaps}[RawFeature={+c2sc,+scmp}]{EB Garamond}

The specimen’s preamble may be a good guide too.