What is the average size of a bitcoin transaction?
Just to give a few exact numbers, the current average transaction size (of all existing transactions until block height 498920) is 522.45 bytes.

If we take into account just the first 100000 blocks, then the average transaction size was indeed 238.51 bytes.

This is a plot of the average transaction size (aggregated monthly):


The peak on mid 2015 is produced by this 999657-bytes transaction.

These values can be computed using BlockSci.