The king is 85mm / ~3.34 inches.  The pawns all have a cutout for a standard m12 x 20mm  (coarse threaded, 1.75mm) hex bolt, and all the other pieces have cutouts for m16 x 20mm (coarse threaded, 2mm) hex bolt. This is a cheap way to give the pieces some added weight.

Pictures of printed pieces are on a board with 2.25" squares.  Both sides shown were printed in econoFil PLA from filaments.ca using standard black and milk white colors. I printed 1 extra pawn and 1 extra queen for myself for each side.

You can use whatever material you want.  I used PLA for the pieces pictured. No supports should be required for these, as the bolt cutout has been tapered at the inside top.  Everything looks pretty good at 0.2mm layer height (pictured prints).

There are 2 versions of the piece models - one with space for the bolt inserts, and one without (solid).  There may be a little tesselation on the solid king and queen - for that reason, I've added the freecad design file so you can export these yourself with higher detail settings.

When I printed these, I had z-seam set to “user specified” straight along the back for the knight / queen / king / rook, so curves weren't interrupted and I had something easy to sand down.