In SCID it's very easy to create your own set of custom pieces.
1) Create a new directory scid_dir/img/pieces/set_name
Substitute scid_dir with the name of the scid directory and set_name with the name you prefer.
2) Create a gif or png image with size of 960x80 pixels.
The image must contain all the pieces of the set, in order: white pawn, white knight, white bishop, white rook, white queen, white king, black pawn, black knight, black bishop, black rook, black queen, black king.
3) Create more images smaller or larger to have board with different sizes.
Remember that each image must contain 12 pieces of square size. So, for example, a set of 50 pixels must be contained in an image of 600(12x50) x 50 pixels.
4) Save all the images inside the directory created at point 1.
5) Start SCID and the new piece set will be available.
Here's a solution by Alexander Klimov, that uses ImageMagick by running the following bash script in img/pieces/Merida/, used here as an example.
~~~~~~~
for s in 25 31 38 46 57 70 86 106 130 160; do
convert -size $((12s4))x$((s4)) xc:white\
-family "Chess Merida" -pointsize $((4s))\
-gravity North-West \
-annotate +0+0 "pnbrqkomvtwl" -background white \
-fill none -draw "color 0,0 floodfill" \
-resize $((12*s))x$((s)) \
Merida_$s.png
done
~~~~~~~~