For subsetting an S1 TOPS SLC you may use the “S1 TOPS Split” operator, by which you may reduce the image to a selected sub-swath. Unfortunately, currently this is the smallest part of the scene you can perform InSAR on. In the coming release users will be able to select at a burst level which will allow to reduce further the size.
You cannot download part of an acquisition. It might be in the future that other option is implemented. Also there is no information on the baseline a priori. However, please consider that the dispersion of perpendicular baselines of S1A data is generally small due to the systematic control of orbital tube.