Power a laptop from 12V Battery using DC-DC Step Up Circuit
1) Safe in what sense ? If done properly then yes, this is safe. But do check the DCDC converter's output voltage and polarity before connecting it to the laptop.

2) No, the laptop will just take as much current as it needs. But it is always a good idea to place a fuse. I would place a fuse of 8 A between the SLA Battery and the converter. This allows the laptop to consume about 90 W which should suffice.

3) the fuse ! Also when operating and the laptop battery is charging, check that the heatsinks on the DCDC converter do not get too hot. Do not place the DCDC converter in a small unventilated box. Give it some space and ventilation so it can get rid of the excess heat.

4) Yes you can change the battery to any capacity you like.