Same as the animal from were type. However, there is a rule in the lycanthrope rules that the base creature and base animal must be within 1 size category of each other.
Were Rat, any Tiny-Medium (assuming dire rat)
Were Bat, any Medium-Huge (assuming dire bat)
Were Rat, any Dim-Small (assuming regular rat)
Were Bat, and Fine-Tiny (assuming regular bat)
Now, that really does not leave many people able to contract lycanthropy from the smaller bat and rat. Guess it is still possible with the rat, but still.
Was trying to explain the limitations in the disease.
A person who was bitten by a wererat would need to be tiny, small or medium to be effected by the bite. Thus a giant is "immune" to the disease.
Similarly, a person bitten by a werebat would need to be medium, large or huge. Gnomes and Halflings, among other races, are "immune" to the disease.
Second section was if the were bat/rat was a normal little animal in animal form. Which reduces who can become a lycan further.
That isn't quite accurate. Lycanthropy can only be passed on to a creature within one size of the Lycanthrope, and it happens when bitten while the lycanthrope is either in its animal or hybrid form, with the hybrid form being the larger of the animal or the base creatures size.
It isn't entirely clear whether it is the current form or the base form that applies when comparing the sizes of the two creatures, but in any event a were-bat human who was in hybrid form would still be medium size and could pass the curse along to small medium or large creatures.
As for the original question, in the special qualities section for change shape it lists bat (presumably just the common bat) and dire rat respectively. So were bats are common bats (diminutive) and wererats are dire rats (small).