Maximum toilet pipe width?
Regulations (if any - they are mostly concerned with minimum sizes) will vary with the LAHJ (local area having jurisdiction) but the simple and practical consideration (if connecting to a sewer) is that your pipe size from the house should be equal to or smaller than the pipe size from the street. 

A reduction in size will be more prone to clogging, and it's your house that will have the clog, not the general public.

There is no real benefit to the public system in limiting the size of pipe in houses - the size of pipe does not materially affect the amount of flow, which is based on number of occupants and fixtures, not the size of the pipe.

Where there is no sewer, the input connection on your septic tank will set the maximum practical pipe size (typically 4" in the US unless special-ordering a larger intake, which is really not called for on any normal-scale single-dwelling septic system.)