How do we calculate the image circle of a telescope at prime focus? I'm trying to find a suitable CCD and telescope combination. So, I want to make sure that the image circle of the telescope circumscribes the CCD. But I'm unable to find anything that tells me how to calculate the same.
Image Circle will vary with scope design. Just about all scopes will cover the field of a 1.25" eyepiece barrel, and scopes with 2" focusers will usually cover the field of a 2" barrel (There are some exceptions where the field is bigger than 1.25" but doesn't cover the full 2"). Basically, you need to check the spec for the scope - and in any case the size of the focuser usally sets an upper limit. (With smaller SCTs and Maks, the size of the baffle tube can also limit the image circle).

Most small refractors for astrophotography will cover an APS sized DSLR sensor.
If you're looking for something to cover a full frame DSLR sensor, the choice gets more restricted - quite a few of the field flatteners/reducers for smaller refractors are only designed for APS coverage.

If you want to go for an unusually large sensor, then your choice is even more restricted - something like Takahashi's FSQ106ED astrograph has an 88mm diameter image circle in native mode, large enough for some medium format sized sensors, or 44mm with the reducer or extender - but there's a high price tag to go with it.

However, if you're looking at large format astonomical CCDs, they're usually very expensive too. Most of the more affordable astro cameras have smaller sensors, so are less demanding on image circle size.