I don’t normally ride on the road but we’re off to club la santa later this year and I’m planning on using the road bikes there but I’m not sure which size to go for, I’m 6’0″ and average proportions(no orangutan arms).
Should I book a 56 or 58?

I’m never sure if I’m 6′ or 5’11”, but either way pretty much always go for a 56cm in a road bike, the critical dimension being effective top tube (560mm) and reach (385mm).

Reason being I’m a typical office dweller with horrible flexibility, and I think most 58cm bikes would start getting too long for me. Have twice hired 56cm bikes (separate occasions but both times a Specialized Roubaix) and both times it’s been fine.

What sort of bikes are the hire bikes?

If you’re renting a bike, ask the advice of the hire centre. We update our sizing chart each year as there are variations across and within manufacturers. For instance, I’m 5’7″ and my own road bike (a Cube) is a 56.

Don’t just go on size, a 56cm bike form two different companies doesn’t size up the same. Ask the hire company.

^ this. Also, bikes of different sizes from the same brand can be very close.
Cannondale 54 and 56 sometimes have the same reach but the 56 has a slightly higher head tube.

Not sure if this Is still the case but it was in 2015 when I looked at them.

If it’s a hire bike, they probably will have a stock stem length and won’t change them. As above, brand will be important too. But the default would be a 56 cm, possibly with a 120 mm stem. However this won’t be any different to a 58 with a 100 stem.

Start with 56 and go from there (I’m the same size as you) – ask the hire company as they will know the sizing of the bikes they carry. 58 will probably be too big. Where companies do have somewhat odd sizing in my experience the bikes tend to be bigger than you would expect for a given size.