The packet size can be controlled by adjusting the number of subcarriers allocated to the user and the number of time symbols. That is, you need to choose them such that the number of bits within the subframe is equal to the packet size (assuming the packet size includes channel coding). You would also need to take into account the modulation order, number of antennas (for spatial multiplexing), the pilots overhead, etc, when calculating the bits.
For the arrival time, we do not explicitly support inter-transmission delay. However, you can introduce a delay by adjusting the nGuardSymbols parameter in the simulation scenario file. But I am not sure if this will give you the desired results, since this will only impact the throughput (due to the wasted guard time / cyclic prefix). I think this inter-packet arrival time is supposed to reflect the frequency of transmissions, and therefore the amount of load that is exerted on the network, which cannot be evaluated directly with our simulator without introducing some resources management scheme.