The Size Instance to Fit Content option is now available in the Layer menu (which means you can also create a shortcut for it).
Our heartfelt thanks to the testers in Sketch community forum (jkilp, eurlichallan, joe, dale, valentin, AACourt, KarlCanBeCool, Dave, leohans, Giancarlo, fab1An, flowxd_io, vattic, rachael, noe, samvermette, srkkrs) for their insights and feedback as we worked on this. You’ll also notice many quality-of-life improvements to Smart Layout, including Layer List icons, keyboard shortcuts, Foresight support, and the ability to pause Smart Layout temporarily on a group or Artboard. Better still, it behaves far more predictably. You can now use Smart Layout with groups, Artboards and Symbols.