Ring size is based on the tube size of your scope. Most rimfire scopes are 7/8" or 1" tubes.
It is possible the spacing of the holes in the action will not allow a variable or other large scope to be used without offset rings. Offsets do not rise straight up like standard rings, but rather have a dogleg that offsets the rings by an inch or so. One or two can be used and the scope mounting options increase by a bunch.
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