Hi Vincent,

the size of the grid in Montage format depends on the number of slices
in your volume. You can calculate it by ceil(sqrt(nslices)).
So you get:
1x1 for nslices = 1
2x2 for nslices = 2-4
3x3 for nslices = 5-9
4x4 for nslices = 10-16
5x5 for nslices = 17-25
6x6 for nslices = 25-36
7x7 for nslices = 37-49
8x8 for nslices = 50-64
9x9 for nslices = 65-81
...

I hope this helps,

Michael