How to set up Amazon RDS parameter group for Postgres?
Amazon RDS documentation provides a table of units for each parameter under the "Working with PostgreSQL Parameters" topic.  Units are also usually included in the description of each parameter in the parameter group edit UI from the Amazon RDS web console.

min_wal_size (Postgres 9.5 Parameter Group Family)


Default: 16
Units: MB
Allowed Values: 2-201326592
Description: (16MB) Sets the minimum size to shrink the WAL to.


shared_buffers (Postgres 9.5 Parameter Group Family)


Default: {DBInstanceClassMemory/32768}
Units: 8kB
Allowed Values: 16-1073741823
Description: (8kB) Sets the number of shared memory buffers used by the server.


Regarding shared_buffers specifically, the documentation has this to say:


  Note that some parameters use units that you might not be familiar with; for example, shared_buffers sets the number of 8 KB shared memory buffers used by the server.