So the harness you have has an interchangable O ring, so you should be able to fit ANY dildo in it by switching out the O ring. The O ring on there is 1.75" so you can go up to that size without having to get a new O ring set. The current dildo is 1.5". So your next size up would be 1.75" or 1 5/8" (for a small size jump).
Suggestions:
Tantus Cush O2 Twilight Realistic Dildo Tantus Cush (1.75")
Tantus P-Spot Silicone Anal Dildo Tantus P Spot (1.7")
Silicone is the safest material you can get, so I would aim for silicone dildos. Tantus makes very high quality products. The Cush is O2 dual density and feels amazing. One of my husband's favorites is the Flurry, another O2 that's 1.5" diameter.
I also second the Joque harness recommendation. If you get another harness, that's one of the best ones you can get.