Can someone explain why there are long and short versions of his letters?
Before printing presses, everything had to be copied by hand, so there was rarely, if ever, a "standard" edition of any work. It just depended on what the scribe copied into your edition. Not infrequently, people would compose a new work (e.g. a letter, a sermon, a treatise) and, as a strategy to give it greater authority, would say that "Famous Person X" wrote it.
Such is the case with the Ignatian letters. In the late Byzantine period, if you were to pick up a Greek copy of the Letters of St. Ignatius, you'd find a minimum of 13 letters in the book. This has often been called the Long Recension. No modern English edition I know of prints all 13, nor do they print the longest versions contained therein.
There were also some other things attributed to St. Ignatius that were floating about in Latin. They were discredited as early as the 15th century, and the early Reformers tried to get all of Ignatius thrown out of the window, because the textual history in Latin for some of these items had no Greek counterpart, as well as a poor manuscript history that only went back to the High Middle Ages.
Anyway, in 1644, a Brit, Archbishop James Ussher, ruffled through a number of British libraries, found some good Latin manuscripts, and published what is typically called the Middle Recension. The Middle Recension had shorter versions of the 7 epistles we know today (Ephesians, Magnesians, Trallians, Romans, Philadelphians, Smyrnaeans, and Polycarp) than those in the Long Recension.
Shortly thereafter, some other dudes dug up several Greek manuscripts in Florence, Paris, and Berlin. These had some differences with the Latin manuscripts, but they were relatively minor. Thus, we have both Latin and Greek manuscripts for the so-called Middle Recension.
That was pretty much it until 1845, when some earlier Syriac manuscripts were found in the British Museum. Both of these Syriac manuscripts had only three epistles (Polycarp, Ephesians, and Romans), and this is called the Short Recension. Lightfoot (and other British scholars) have argued that the Syriac manuscripts were merely an epitome of the Greek, whereas some Germans have argued that the Syriac manuscripts are actually more ancient. A Greek fragment of Smyrneans does exist that pre-dates even the Syriac manuscripts. Today, many scholars accept all seven epistles as authentic but some think those Syriac versions may perhaps contain better and more ancient versions of those three letters. Thus, a lot of modern editions use the Syriac manuscripts for Polycarp, Ephesians, and Romans, instead of those from the main Greek manuscript, which is in Florence.
Hope that's clear as mud. Welcome to the wonderful world of manuscripts.