How do i work out whats the best size road bike i should buy?

1st answer is very good. I can add a general place to start.

Road bikes have a general rule. You need to be able to stand over the top tube with about a 1" clearance. Besides the obvious reasons of safety, a bike in that range will have a general top tube length that will be close to what you will need.

You will find many bikes that fit your standover requirement now look for the one that has the length you want. Remember you can adjust this length with a longer/short stem too.

If you're new to road bikes you may find that the reach to the bars is very long on performance and racing bikes. You may feel the bars are too low too. Stems can be flipped over to raise or lower the bar. Try different bikes to see how it works for you.

Again if you're new to performace road bikes you may not like what it feels like. Your opinion on fit will change over time if you into performance. A new road biker will not like my bike because while my reach is relitively short my bars are low. As you put on miles you will think, wow, my bars are too high now or my reach is too short. This is normal, people grow into what they perceive as an extreme position at 1st.

If you're looking for a road bike for casual or infrequent rides you'll want a shorter reach and a high bar position. A performace bike and a casual ride may not be the same bike.

As you listen to advice understand the context of the advice, racers may tell you a race setup is the only way go, that may not be what you want. Understand what you want to do with the bike before you buy.

If you have the availability, head down to your local bike shop and try some out. Bikes are more universally the same than say... shoes, but they can still have variances. Plus, this way you can get on and see how the sizes handle. If you cannot do that, do you research online first. You will actually get more concrete answers using your search bar than asking in this forum. Plus, the reference list you will receive will prove more useful as well.
Good advise here. Make sure the folks you are talking to understand your usage; road bike for faster commuting, competitive racing, fitness, or recreation. (the bike that fulfills the intended purpose best gets ridden the most)

Also if you have any physical issues- bad neck, etc., declare that.

good luck and tail winds.

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