I'm 6'4 and would like to know roughly what size i should be looking at.
rode a friends 56cm frame yesterday which felt fine but drop bars felt too low (when in 'racing position).
Hope i don't need to buy a garden gate as i hate big looking bikes.
will a 58cm suffice? Obviously I will try for size before i buy.

nah you indeed want a 60cm minimum id say, if you dont like the look of the big bikes, get a compact frame, mine looks tiny!

How will I know if it's a compact and who makes them? Sorry for dumb question but I'm a newbie!

Only way to tell is to try some out at a shop, road bike sizing is more critical because of the time spent in the same position 😉

The new 'sportive' end of the market is probably what you're after as they usually have longer head tubes so raise the handlebars. You can acheive the same effect on a standard race bike but it looks silly with lots of spacers.

I was always led to believe you needed 9 inches less than inside leg measurement. Not sure if that is correct though

Which leg?
The only way is to go to a shop and get fitted/try some out.
I imagine 56 would be far too small for you.

I'm 5'11 and got a 56 from my LBS the 58 didn't feel right, however when I went to a new shop owned by a mate he spent an hour setting up my riding position and said he would of recommended a 58 also got the same opinion from a Specialized Concept Store.

any where from a 60 to a 66 depending on leg length and arm length you really need to try them out though

Decide what you want the bike for – racing, sportives, touring etc. Get a bike fit! Try several bikes for size. Take as many as possible for test rides.

The following has some useful guidance:
http://www.competitivecyclist.com/za/CCY?PAGE=FIT_CALCULATOR_INTRO

I'm 6'3" and have ridden everything from a 58cm to a 62cm. Current bike is a 61cm but that's a compact frame. Just goes to show how sizes vary between brands. Traditional frames (ie horizontal top tube) are relatively rare these days, most companies go for compact or semi-compact geometry.

It's not the seat-tube length that's the issue particularly, more the top tube and hence the reach.

The only way to find out is to go to a decent roadie shop and try a few. I'd be guessing you need a 60cm minimum and you might find those hard to come by in bike shops, maybe try a shop with a bike-fitting/measuring jig.