How to determine widest rear tyre a bike can take

[Gate closer]

I recently moved to the coast and now have need of some mud tyres. It never stops raining in Toti (not complaining) My bike is a Momsen Vipa AC with a 2x10 setup and with the current Honey Badgers on I seem to spend a lot of time going straight off the track into bushes or looking for traction. The question is how do you measure the maximum allowable width of a tyre that will fit in the chain stays bearing in mind the build up of mud that takes place. Is there a rule of thumb that you need X amount of clearance between tyre and stays and what would be a good tyre for these conditions? 

 

Thanks