Is there a need to size differently for OLTP and OLAP systems? Why ?
OLAP systems involves more of "Select" queries and hence one needs to have bigger buffer pool to cache more data pages. Otherwise, OLAP systems will have lots of disk IO.
OLTP Systems normally comprises of insert statements, random I/Os, smaller transactions and don't have huge select statements. As the types of workloads are different, a slightly different formula for different systems. Some OLTP systems also have huge reports causing massive select statements, but the formula can serve as a general guideline.