Secondary index memory footprint

[Sumit Nigam]

Hi,
I am trying to estimate what (if any) are the implications of accumulating data 
in phoenix secondary index. I have a secondary index on 3 columns and would 
like to know if anyone has an idea of how to estimate memory footprint of 
secondary index (if any) based on number of entries in data table. The index is 
global and not local.
Plus, what is the impact of setting IN_MEMORY=true when index is created? 
Thanks,Sumit