What is the size in cm of the squid MEG used in BST

What is the size in cm of the squid MEG used in BST?

It depends on your MEG system.
The geometries of the sensors supported in Brainstorm are defined in file coil_def.dat, and shared between Brainstorm, MNE-C and MNE-Python:


https://mne.tools/0.12/manual/source_localization/forward.html#chdbdfje

These links where already posted in reply to one of your earlier question.