I'm a big rider. The theorists tell me I should be riding 44cm bars, maybe even bigger. Actually I prefer 40s, or 42 max. Nobody can tell you what will feel right to you.

Having said that, unless you feel your breathing is restricted because your chest is closed I'd give the 38s a try for a while if I were you. You might find that like me, you come to prefer a slightly narrower position. If after a few weeks you still don't feel right, move up to a set of 40s.