Why the overlapping area become smaller for: level-0 RAW to level-1 SLC to level-2 GRD?

and can I use level-0 RAW to calculate river flow velocity by feature matching of SAR data?

RAW data is so blurred you will most probably not even be able to see the river.

alright, so I should change to use other satellites’ SAR data?

Level-1b SLC focused images is what you should using, with all sensors, if you are interested in high resolution.

OK. Does the level-1 SLC data point have precise geographical information?Is it gridded data?