What is the size in cm of the squid MEG used in BST?
It depends on your MEG system.
#
# MEG coil definition file
#
# Copyright 2005 - 2019
#
# Matti Hamalainen
# Athinoula A. Martinos Center for Biomedical Imaging
# Charlestown, MA, USA
#
#
# <class> <id> <accuracy> <np> <size> <baseline> "<description>"
#
# struct class id accuracy num_points size baseline description
# format '%d %d %d %d %e %e %s'
#
# <w_1> <x_1/m> <y_1/m> <z_1/m> <nx_1> <ny_1> <nz_1>
#
# struct w x y z nx ny nz
# format '%f %e %e %e %e %e %e'
#
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https://mne.tools/0.12/manual/source_localization/forward.html#chdbdfje
These links where already posted in reply to one of your earlier question.
If think you can read only the channel information from the FIF file with the Brainstorm function.
However, it could be easier to have all your data (sensors information + data) in the same file format.
Or even easier, if you don't really need to have the data written in .fif (for processing in MNE-python for instance), then simple write your own code to integrate the sensor geometry directly in a Brainstorm channel file.