TempDB's Size and the Buffer Pool Source:


  Next, if you can give tempdb its own disk, then configure it to almost
  fill the drive. If nothing else will ever be on the drive, then you’re
  better off setting it to be larger than you’ll ever need. There’s no
  performance penalty, and you’ll never have to worry about autogrow
  again.


Doesn't tempdb fill the buffer pool, so wouldn't this have an affect on things like the page life expectancy?  Or does this only have an effect if tempdb is actually in use?

Another way of asking this question - does the size of tempdb matter to the buffer pool or does the used size of tempdb matter to the buffer pool?
does the size of tempdb matter to the buffer pool or does the used size of tempdb matter to the buffer pool?


No tempdb physical size does not matter (you should properly (equally size) you tempdb files (not more than 8) with TF 1118 and 1117 enabled). 


  Or does this only have an effect if tempdb is actually in use?


A system with heavy usage of tempdb, you will see many dirty tempdb pages in the buffer pool.

Remember that the Buffer pool is used for caching index and table pages as they are modified or read from the disk. Also, there is a concept called buffer pool disfavoring which prevent pages from a different database being flushed from the buffer pool when a large operation occurs (e.g. bulk activities). 


  so wouldn't this have an affect on things like the page life expectancy? 


Read up Page Life Expectancy isn’t what you think…