Hi Marnie,

That is not toooo huge a cluster size, assuming those are
1x1x1 mm voxels. Are they? I am used to seeing such a
number as a minimum volume in microliters, though maybe
for a more gentle threshold. Is your mask possibly at an
even higher resolution than cubic millimeters?

. . .

For comparison, I just ran that same command on a 1x1x1
dataset of a normal EPI mask and got a 0.05 correction
at 233 voxels.

Running it on an anatomical mask (so bigger than the EPI
mask, but still appropriate) shows a cluster of 367 voxels.

Using the entire dataset as a mask, I get 497 voxels. This
would be a mistake and the mask goes far outside the brain.

---

Based on those numbers I expect that you are either using a
dataset that is inappropriately high-resolution for EPI-based
results, possibly with voxels even smaller than 1x1x1.

So what are the dimensions and voxel sizes of your mask?
How many voxels are actually in the mask?

- rick