the needle type/size is dependant on the thread being used-
some people use (and love) bottom line or other 'bobbin' threads- some use the same thread top & bottom- it is (generally) easier to have good tension when the threads are the same- or at least the same weights in the top & bobbin.
you can visit Superior Threads - they have free charts you can print off that tells you the different threads- their weights & uses---and what needle should be used with each one. The charts are a good resource!