It doesn't matter what we think on what size that may work or not.
In your first post you stated you wanted a 12x4x3, if we all said "ya go for it you will be good" Would it be right for you to assume that you would really be good. No it would not, it really doesn't matter what you think will be good either.
Have your mobile home inspected by the right person. Someone with the qualified credentials to tell you how you can beef up your floor properly, and have them or another qualified person give you a estimate on how much dead weight it can support. Then once you have that magic number, you can go and figure out how big of a tank you can build with the Tank calculator for size and weight, remember to factor in the weight of the glass and stand and among other things.