Is it a bad practice to separate the unit tests for a class?

My Classes normally have about 5-20 methods maximum, that implies that the class that has the unit tests has about the double of methods (counting dataProviders, setUp, ...)

I have thought to separate the tests in 2-3 classes. Is it a bad practice?? Is good to have all the tests for a class in only one, although the test class grows in vertical?