Take a look at this:
This should help - but basically the O-Ring on this particular harness has a minimum diamater of 1.25 inches and a maximum of 2.25 inches.
This means any dildo with a circumference of 3.93 inches up to 7.07 should be suitable (as long as it has a flared base).
You just need to have a browse through the dildo's, see what takes your fancy and just check the measurements, which LH kindly provide on all their dildos.
I hope this helps and happy dildo shopping!
It should work with the more realistic ones (http://www.lovehoney.co.uk/product.cfm?p=22814) for example - you'll just need to insert it by pinching the flared sucker and feeding that through the O-ring, rather than sliding the shaft through - if that makes sense!