- What's the minimum / maximum amount of space can be recognized from the Mikrotik? Can I expect some problem with that?
You'll have to try, but I'd expect ROS will support all "normal" SD card sizes.
- Once the internal storage is over will start writing on the MicroSD or ?
No, both storages are completely separate. ROS won't ever use external storage for its own use except for a few rare exceptions (which still need to be configured explicitly, e.g. logging to disk). External storage comes handy for add-ons, such as containers or userman (again need to be configured explicitly to use external storage for their use).
Just beware that ROS devices aren't particularly good NAS devices even though this vastly improved with introduction of rose storage package.
- ext3 or ext4 better?
I don't know if ROS supports ext4. But it really doesn't matter, the safest thing to do is to format card within ROS. Otherwise ROS might have problems using card even if it seems to be formatted (in normal linux machine) according to ROS expectations (which AFAIK are not documented anyway).
