I have an XDm40 and a HK45. I carry the HK45 as my only carry gun. I just compared it to my XDm40, and it's just a smidge bigger, especially with the exposed hammer. The HK45 works for me for CC, and I wouldn't hesitate to carry the XDm. That said, I'm tall and used to carry a Kimber, so maybe that makes a difference.
I guess my point is that while a smaller pistol is better for some for CC, I wouldn't completely discount carrying a full size pistol on a daily basis if you're comfortable with it. Regardless of size of pistol, you'll want a good holster and belt, as it makes a big difference (especially for full size pistols).
i'm a short 5'8" and with a square build (so anying on my belt will stick out) ontop of that with a G35 concealed. Carrying a xdm will just take a little bit of creativity and a lot of confidence to not attract attention to it (adjusting your rig, walking with a limp, being overly protective of your strong side when its not warranted, etc).
I swear wasn't there already a thread on XDM for CC?