The physical data model is a network model and illustrates the database tables, keys, fields, data types, relationships and cardinality.

If you have followed the methodology to this point and you have have produced a "Structur Model," you can clearly see that what remains to "database design" is defining the physical characteristics of the data using the tools associated with the DBMS.

Primary Keys are always defined as a unique, indexed, self incrementing interger."
Seconday Keys may have any definition and are always indexed.
Data Items may have any definition and may be indexed (rare -- special monthly runs).

Defining "Blocksize" is also a function of design. Blocksize refers to the number of bytes access by a single database call. This "physical record" -- a block -- contains many "logical records" -- table rows. Common blocksizes are 8K, 16K, 32K and 64K and usually default to an installation standard.