If you're using a harness then you'll probably be using the dildos with a partner, correct? In that case you probably want a dildo that's comfortably within your known limits so you can both relax and enjoy the sensations rather than being worried about being hurt/hurting your partner.
That said I strongly recommend working up to at least a 1-1/2" diameter dildo before going to harnessed dildos. Smaller dildos are more flexible and harder to control, and the dildo should have at least 5" insertable so it doesn't pop out unexpectedly.
I personally like the 1.75"- slightly over 2" range. A dildo of this diameter does a great job stimulating the prostate and lets me feel nicely stretched/filled without needing too much warmup. Silicone dildos are also easily available in this size, another consideration (silicone is clean, safe, and as a soft material less likely to cause injury during partner play).