Hi Manjari Sinha,
Let me try to answer your first question:
You can find the size of the info cubes in our BW production system from ST14. Here is the procedure how to do that. Not sure if you have enabled/access to ST14.
Goto ST14 -> Business Warehouse -> Schedule Job -> Give the client
number, Analysis Title and in analysis scope Goto Basis and Other and
select DVM startegy, in Documents select the obj type as PSA time
distribution and give the value as X for Document no and schedule the
job to get the top infocube tables of the BW system.
BR
Prabhith