Is my ceiling too low?

you're fine....don't hit your head

Your lower ceiling is only a problem in that you're getting more reflection in time vs. direct sound from your speakers, all things equal. If you sit closer to your speakers, this isn't an issue so much. Also, more "controlled focussed speaker designs", dipoles, Dappolito's, etc, tend to cancel out off axis (vetical) frequencies some what, minimizing ceiling/floor reflections...this helps. Otherwise, you need ceiling treatments more likely than higher ceiling. Again, if you sit closer to your speakers this negates these issues quite a bit, and helps.
On the other hand, if your ceiling height ratio,vs one of your length modes is within 5% of each other (7.8 and 15.3 in your case), you'll also have doubled up bass modes in certain ranges, which will definitely compound problems acousticall as well!! Proper speaker/seating becomes even that much more critical, and even some of this new EQ stuff on the market, like Rives PARC, or more DSP bassed stuff that's now arriving. Good luck

You are right, I didn't notice the length/height is so close to 2! This will be a bigger problem... Anyone can tell which case is worse: length/heigth=2 or length/width=2? or the same?

I have a family room (LxWxH=19x13x7'9) in the basement can be used as a hifi room too(but will cause more money to build two doors to close this room. Is this room better than the other one(15'3x12x7'9)? Is it too big for my system? Thanks.