That seems small to me (I tend to sit in the middle of theaters as well), but I tend to prefer a larger screen, particularly for a theater. I suggest you sample a similar distance on a similar sized screen and decide for yourself. My guess would be that your salesman has a screen in inventory that he wants to sell in that size.
There are several screen size calculators online if you google it. You will find different recommendations from SMPTE and THX, for instance. The bottom line is, preferences vary, so you need to get some viewing experience yourself.
In the years of installations that I have done, I have found that few clients are unhappy with larger size screens.