Many years ago I was measured and told I should use 38cm bars . . . so I did. Not knowing any better, I thought they were alright. That is, until I was building up and bike and (much like the OP) only was able to get some 40cm bars.

Wow, what a difference! I liked them so much better I've stuck with 40cm ever since. I've tried 42's on someone else's bike for a short time and instantly felt they were too wide. So yes, sticking with 40's regardless of the "measurement" of my admittedly wimpy shoulders.

Rick / OCRR