Relation between sampling frequency and low pass filter order.

I want to filter a signal with front end sampling rate 200MHz and decimation rate 128 using low pass filter fir1() in matlab. It is known that the message is around 100kHz frequency. What should be optimum order of the filter to be taken? Is there any relation between sampling rate and the order of the low pass filter. I am dealing with ATC signal collected using loop antenna array.

Thanks in advance for help.