Hi Beate,

That filesize limit error means the output dataset is more
than 2 GB, and that you are using a 32-bit operating system,
which does not allow creation of files that big.

The reason -master=epi works out is because of the resolution
of the data (and therefore the size of the dataset).

If the mprage is basically 1 mm^3, and the epi is 4 mm^3, then
one volume of mprage data is 64 times as large as the same
volume of epi data. That's why you were running out of RAM,
the EPI data was being resampled to a fine grid, making the
dataset very large.

It is not necessary to resample the EPI data much, if at all.
Leaving it at the EPI resolution is appropriate. If you want,
for some reason, to make it a little more fine, then something
like '-mast_dxyz 2.0' would work, too. That would "only" be 8
times as large as the 4 mm^3 dataset.

- rick