You'll likely want to set them all at "small" so they can crossover to the sub.
I looked up your speakers on Amazon and the specs cited a low frequency limit of 34Hz for the mains, 35Hz for the surrounds, 53Hz for the backs and 48Hz for the center. An 80Hz crossover point will probably work just fine. It will give you enough overlap to achieve a smooth transition between the subwoofer and the other speakers.
If you can set the crossover point on the back channel independently of the others you might bump that one up to 100Hz. the claim of 53Hz low frequency cutoff frequency on a 5.25" driver with a rated efficiency of 89dB seems a little optimistic.
MRX-520. Mirage OMD-5: left, right & surrounds. Mirage OMD-C1: center. SVS-SB-2000: subwoofer.
Intel NUC running JRiver 21 as DLNA renderer. Samsung BD-C6900. Roku 3.