Downloading sentinel image

is it possible to download the sentinel image by shapefile of one’s study area
in order to reduce the image size

Unfortunately not, you should download the entire tile product, and then after some processing steps , it’s possible to clip the AOI, and this with regarding to S1 SLC image. However concerning GRD S1, you could subset directly after downloading the entire tile. Similarly to S2 tiles , it’s possible to subset once the tile is downloaded.

thank you so much