How do I determine the appropriate size for my tablespace when importing a .dmp file? I have a 14g DMP file. How large should I size my tablespace to import it into an Oracle 11g database?
At least 14 GB. 

Maybe more if the dump file is compressed.

Maybe less if the tables are compressed.

A bit more or lot more if there are indexes.

Even Data Pump will miscalculate the required space in case of compressed tables.

Index data is not stored in the dump file, only index definitions. You can have a 14 GB dump file with 14 GB table data and 0 index definitions or hundreds of index definitions, and building those indexes may consume a lot more storage than the base tables, or just a fraction of it.

So the correct answer in my opinion is, test and measure it.