That a pair of reidi are not appropriate for an 18 gallon tank (or are you planning to get them a different tank?) That clownfish are generally not recommended as tankmates.
Reidi can grow quite long -- 8" to 10" wouldn't be uncommon. Not the largest seahorse, by any means (pots and ingens, and possibly the kellogies, duke it out for that title), but they aren't small.
If the seahorses come from a good source and are properly taken care of, they can live five or six years, or more, but the majority do not reach that age.