This is really a question that is almost impossible to answer. If you are talking about your tank with 5 Mollies then you need 20 gallons but if for just a beginner who is not sure what the fish are that they want a 10 gallon is a nice choice. A good betta tank for those who do not have much space is the 5 gallon. So you see, it really depends on the purpose you have for the tank.
In my opinion the best size of tank for a beginner is a 20--it's not to huge or hard to handle, but it's easier to maintain tahn a 5 or 10 since it has a larger volume of water so it is inherently more stable. If you make a mistake in a 5 or 10 gal it has a lot more impact than a 20. However many people who do not have the room or money start out just fine with a 5 or a 10.
For you Cowizzle, you need at least a 15 gal tank to house 5 adult Mollies, and a 20 would be better. A 2.5 is really too small for any tropical fish.
I started with a fishbowl I now have a 55 gal. although because of the addiction my fish have for me withdrawl is not an option so I must upgrade agn I thk I am goin to get a 150 we'll see I know once I get to the pet store I will see more fish and wow when does it ever stop?....Clint