how to estimate the cut off frequency in the filter designer tool?

I'm using a filter designer tool in Matlab to calculate root raised cosine filter for windowing operation in OFDM... according to the standard I'm working on they specified the sampling frequency and bandwidth only and there is no information about the cut off frequency of the filter.. the problem that I don't know how to estimate the cut off frequency of the filter from the given parameters to obtain accurate coefficient values

also I would like to know the relation between the order of the filter and the number of filter taps... I guess one exceed the other by value one but I'm not sure

also I would be grateful if someone helped me to convert the output (signed) value from floating point to fixed point in binary form.. I searched for alot of sites to get the procedure but I didn't get accurate and simple steps