One way to estimate the upper limit of the magnitude is through observations and measurements. According to the article Seismic Wave Interactions Between
the Atmosphere - Ocean - Cryosphere
System and the Geosphere in Polar Regions, a couple of difficulties with determining how large ice-quakes can get, asides from the regions not monitored, are due to the sources of the recorded motion and delineating cryoseismic and tectonic observations. From the article:
Such cryoseismic sources include the movements of ice sheets, sea-ice,
oceanic tide-cracks, oceanic gravity waves, icebergs and the calving fronts of ice caps. At
times, it can be hard to distinguish between the waveforms generated by local tectonic
earthquakes and those of ice-related phenomena.
However, in the Scientific American article Long-Overlooked "Ice Quakes" Data Provides Insights into Calving Glaciers, they can be identified by their location and unique waveforms identified relatively recently.
According to the first article, unlike tectonic events, cryoseismic (= ice-quake) events are seasonal and heavily climate-dependent, with the article detailing that observations have shown that in polar regions
microseism amplitude is attenuated during local winter for both primary and
Observations of body-wave magnitudes (Mb) between 1987-2007 in an Antarctic station were up to 6.5-7.0
A recent observation are the related 'glacial quakes', which generated log period surface waves (greater than 25s) which, according to the article, is
equivalent in strength to those radiated by
standard magnitude five earthquakes, and were observable worldwide. The glacial
earthquakes radiated little high-frequency energy, which explains why they were not
detected or located by traditional earthquake-monitoring systems. These events are two
magnitude units larger than previously reported seismic phenomena associated with
glaciers, a size difference corresponding to a factor of 1,000 in a seismic moment.
Similar observations have been made in the Greenland ice sheet.
NOAA has some sample cryoseismic traces on their PMEL Acoustics page Icequakes.