A quick Google shows that 100mg of diamorphine HCL dissolves readily in 0.2ml of water. Therefore 500mg would require 1ml. However, you're probably best using a 2ml syringe to account for cut etc..
I'd go with the standard Easy Touch 28 gauge 1cc half-inch insulin syringe. If you don't know what a 10 pack of these looks like, heres a picture .. if u have any more questions please ask
How big is your habit? 0.5g sounds like a lot to bang at once. I used to have at least 1g @ day habit and did some pretty heroic shots but never that much so unless you've got an insane habit you're putting yourself at a lot of risk of od.