I concur that you can live very comfortably on that salary in Frankfurt.
If the gross monthly salary is 6900 €, the net ("netto" in German) or take home salary will be roughly 4000 €. The exact amount depends e. g. on whether you choose public health insurance or opt for private health insurance, which is only possible above a certain income threshold (but you are well above), and for younger people without chronic conditions it is cheaper than public health insurance.
Regarding apartments, for +/– 13 €/squaremeter you can get a nice centric apartment. The only difficulty might be that the overall supply of decent, centric apartments isn't too big. You probably need to turn to a real estate agent (Immobilienmakler), and they'll ask for 2 months rent as remuneration after finding you something (See related thread about "Makler scum"

). So the initial housing costs can be considerable (2-3 months rent deposit, sometimes there is no built-in kitchen in the apartment), but life here is much less expensive than say in London.
Frankfurt is a great hub for travels (if you are fed up with the Frankfurters), and it is pretty international for German standards. More than one fourth of the people here have got a foreign background, and in general "we don't bite", as we say in German.