Try this:
r = Norm[{x, y}];
Plot3D[r^2, {x, -Pi, Pi}, {y, -Pi, Pi},
Ticks -> {{{-2, " -2 "}, {0, " 0 "}, {2, " 2 "}}, {{-2, " -2 "}, {0,
" 0 "}, {2, " 2 "}}, {{0, "0 "}, {10, "10 "}, {20, "20 "}}},
TicksStyle -> Large]
giving this:

Have fun!
Then try this:
r = Norm[{x, y}];
sp = Spacer[20];
lst = {{{-2, "-2"}, {0, "0"}, {2, "2"}}, {{-2, "-2"}, {0, "0"}, {2,
"2"}}, {{0, "0"}, {10, "10"}, {20, "20"}}};
Plot3D[r^2, {x, -Pi, Pi}, {y, -Pi, Pi},
Ticks -> Map[ReplaceAll[#, {x_, y_} -> {x, Row[{sp, y, sp}]}] &,
lst], TicksStyle -> Large]
and play with the argument of the Spacer
.
