has a radius of at least 4.5 light years. First images were circa April of 2002 and the two below are from December 17 2002 and September 9, 2006.
I don't know if this is the largest biggest, but I'll mention it to get things started and because it's so darn cool to look at.
Sources above: V838 Mon HST, below: V838 Monocerotis light echo (HST, September 2006)
Figure 2 of the Nature paper describes the preservation of the actual light curve (history) within the structure of the light-echo shell:
"FIGURE 2. HST images of the light echoes
The apparently superluminal expansion of the echoes as light from the outburst propagates outward into surrounding dust is shown dramatically. Images were taken in 2002 on 30 April (a), 20 May (b), 2 September (c) and 28 October (d). Each frame is 83" times 83"; north is up and east to the left. Imaging on 30 April was obtained only in the B filter, but B, V and I were used on the other three dates, allowing us to make full-colour renditions. The time evolution of the stellar outburst (Fig. 1) is reflected by structures visible in these colour images. In b, for example, note the series of rings and filamentary structures, especially in the upper right quadrant. Close examination shows that each set of rings has a sharp, blue outer edge, a dip in intensity nearer the star, and then a rebrightening to a redder plateau. Similar replicas of the outburst light curve are seen propagating outwards throughout all of the colour images."
From Astronom. J. 135, 2, 2008 or ArXiv