220 is a roll film, essentially the same as 120 film, just a longer roll. It's 6cm wide, and picture format depends on the camera/lens, varying from 6x4.5 up to 6x7 (and beyond, for panoramic shots).
So for use with your telescope, it'd need to produce an image circle that'd cover at least 6cm.
Minor point, but the immersion is the heating element that extends to within the tank, short for "immersion heater".
The tanks generally also have a heat exchanger within that's hooked up to the wet central heating system, that way you can have 1 boiler so both central heating and water heating.
So for use with your telescope, it'd need to produce an image circle that'd cover at least 6cm.