why I can't use many BEs one for each department and link all of them in one FE is that bad idea ?
you can, but you won't be able to have relationship rules between the two db's and potentially make it slower.
I'm going to drop out of this discussion for now, but you are obsessing about the db size. Slow performance and size are not related
. What matters is a well constructed back and front ends and decent network performance.
You have asked for advice and basically ignoring it in favour of your original two thoughts to resolve the problem.
investigate all of these and answer them
- slow network (easy to determine) - not answered
- lack of indexing - not answered
- poorly designed queries - not answered
- poorly designed forms - not answered
- users using same front end (an easy win to fix) - answered, each user has own front end
- backend needs compacting (another easy fix) - not answered and could solve your size issue