Well, there are a few things I would advise you to consider.
For starters, everyone in the old testament would be middle eastern. So he'd look very much like the others, or this would (likely) have been pointed out.
Second, there's the psychological aspect. The story is that, in order to prevent an all out battle, it was offered that Goliath would fight any man of their army one-on-one. This could indicate that he was just a large man (6 feet, or 2 metres tall). Perhaps one with an added deformity (the six fingers, which does happen naturally, if infrequently).
Third, there's the plausible implication that Goliath is a Nephil, though I've not come across any that support this theory. But, if you want to go that route, you can see what Nephilim looked like, and base it on that.
Fourth, there's the fact that Goliath was a soldier in the army. Though it stands to reason he'd stand out anyway, he wasn't a general so he needs to follow the same rules as all the other soldiers. Whatever he wears, or is allowed to wear, needs to also apply to everyone else. So if you want the accurate version, he needs to stand out as little as is possible from his compatriots.
Hope this helps.