How does the OFDM system receiver decide the OFDM symbol length?
First of all, the timing, phase and frequency offsets of the channel are removed. After synchronization, the cyclic prefix is removed, to avoid ISI between OFDM symbols. Now because the transmitter and receiver agree upon a common standard before the communication begins, the receiver knows how many samples to consider as a symbol.

Generally, the the number of samples in an OFDM symbol depend upon the bandwidth.