In general, you should roleplay the limitations of your form.
Spiders should not speak common, rats should not drag things that are larger than they are (with the exception of anything already in their inventory which is "magically" melded into them), and so on.
The exception to this rule of thumb is that you're allowed to drink potions as an animal until further notice, consider this just a curious magical quirk in the absence of a good rational explanation.
But, yes, in general - roleplay the limitations of whatever form you choose.