whats a true concensor? You'll need to do the heat balance and then do the old Q=U A deltaTlm. then look for exchangers with that area. Then once you find some in that area, look at the arrangement and run pressure drop calculations on each side if its acceptable. Since its a condensor, see if the exchanger nozzels can be arranged to allow for a liquid seal.
A truism that is said is that condensers are heat exchangers but heat exchangers aren't necessarily condensers. Condensers are designed with that purpose in mind and since most vapor streams have non condensables present the condenser has to be designed to accomodate that need. Heat exchangers that don't have air removal sections sometimes function adequately as condensers but then again, sometimes not.
OK then, we currently use a vertical "heat exchanger" of surface area 130m2, which achieves what we need.
If a "ture condenser" horizontaly mounted is more efficent, do I need one of a small area? If so how much smaller. Or do I just go for same size, given we would like to improve?
Why are you making the assumption that a horizontal condenser is more efficient? I used to deal quite a bit with vertical reflux condensers, and when we wanted to increase condensation (speed up the process), we replaced them with many more, longer tubes still oriented vertically and had great results. And removing non-condensibles still was not a problem with the vertical orientation.