Let's tackle the two questions you have separately:
The Context for Elster's "Unbelievable" Surrender
From the American perspective, the number of troops involved in the surrender was large. U.S. experience in Sicily in the summer of 1943, for example, was that German surrenders were rare and relatively small in number, versus large numbers of Italian surrenders (See Atkinson's Day of Battle here (Google Books: http://goo.gl/C7V3s) for example.
Elster's forces were not in a position which dictated immediate and urgent surrender, as indicated by the surprise and disbelief at the surrender "without a shot" in the 2 October, 1944 Life magazine article after the surrender which popularized this particular incident (Google Books: http://goo.gl/ob1If). While this surprise may be natural on the U.S. side, in fact, this hides the fact that Elster was not in command of a coherent body of organized units. Instead, "Elster's column" was a ragtag group of extremely mixed units (including Indian volunteers in the Indische Legion, and Ukrainian and other volunteer forces) who were following orders to retreat from the southern French coast and nearby areas. More than half of the total 100,000 or so escaped. It was a long, some 30 mile string of forces under daily harassment from Allied air attack, and what remained of it surrendered when Elster lost contact with his screening force (see Retreat to the Reich by Samuel W. Mitcham, p211)
Largest Number of Germans to Surrender Before Elster
The question of what was the largest number of Germans to surrender is a bit trickier. This might be counted two ways: Total troops who surrendered at one moment as an act of their overall commander, or troops of various units who surrendered in the course of some chronologically limited "battle" and geographically delimited area. The latter is probably more important from a broad historical perspective, but very arbitrary to define, while the former has little historical significance while being easier to determine.
- If defined as troops collectively surrendered by their commander, the answer is, as pointed out by (@Kobunite) most likely the Stalingrad surrender of Field Marshal Friedrich Paulus on 31 January, 1943. Anthony Beever quotes the number 91,000 men, and 22 generals (Beever's Stalingrad, p396), but elsewhere notes that this number was "proclaimed by the Soviet government" (p399) and thus should be seen as highly suspect. Also, there is no clear indication how many of these 91,000 include German units that surrendered long before Paulus officially surrendered (examples of this on p360), and unclear how many of these include non-Germans (several thousand Romanians, for example). It seems likely, however, that whatever the exact number, it far exceeds the number, and certainly the importance of the Elster surrender in 1944.
Other places, you may find more details:
- Soldaten hinter Stacheldraht. Deutsche Kriegsgefangene des Zweiten Weltkriege by Rüdiger Overmans