How do I smooth a low-poly mesh without shrinking its volume? I am trying to turn a low-poly character mesh into a smooth high-poly for better sculpting, is there a good way to do that? Subsurf modifier makes the character skinnier.

example left cylinder to right:
There is actually no need to subdivide mesh before you sculpt.  If you go into sculpt mode with your character selected, you can then select Dyntopo > Enable Dyntopo on the toolbar.



This will automatically subdivide you mesh as needed while you are sculpting.  You can change the Detail Size to a lower number for more detail.



Here is what Dyntopo with a detail size of 3px does to a cube that only had six faces before I started sculpting:



As you can see, with dyntopo enabled, Blender only subdivides the area that is being sculpted, instead of subdividing the whole mesh.