Specify the number of external port lines that you need.
Sometimes pins can be shared.

Calculate the RAM / EEPROM that may be needed.

A single chip solution can often avoid any external memory bus.
Your physical chip size then depends on your GPIO requirements.

You seldom need super fast access to all RAM or EEPROM, so it is also worth considering external SPI or I2C chips if you can't find quite enough memory in a single microcontroller.

Your options are very wide. Getting your design requirements down on paper will speed you through.

David.