Dimensions must be in powers of 2
e.g.:
2 X 2
4 X 4
8 X 8
16 X 16
32 X 32
64 X 64
128 X 128
256 X 256
512 X 512
These are all dimensions in powers of 2
100 X 100 is not.
WrpEdit will only accept a dimension of 256 X 256, but you can import a smaller 100 X 100 dimension into a new 256 X 256 map.
Planck