TempDB's Size and 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…