Oh yeah for sure, 1/2cc rigs are plenty large enough. Especially with just one bag of dope. Remember that no matter what, based on its high solubility in water, all of the heroin is going to dissolve (look at those stats that amapola posted). In 1/2ml, a little over 300mg of actual diamorphine can dissolve.
Because heroin is very cut, and even if it were 100% pure, a .1g bag will surely dissolve all of the heroin (anywhere from 0-100mg). Since all of the heroin will readily dissolve in cold water, the more water you add, the more room becomes available for cuts and impurities to dissolve.
So a 1/2ml syringe is plenty large enough for many bags of dope and more water is not necessarily better.