Just ordered a 18" Barrel with mid length gas system.  What would be a good length to look for on a free floating hand guard, something that covers the gas block.  I was looking at Midwest Industries Gen2 SS One piece but not sure what length to get.  I'm just not sure what length will look just right.  

Also anyone have any other free floating hand guards the like.  I like this one because it has the one top rail and then I can add smaller rails to the sides and bottom for more attachments if I choose the route.  Going for a more sleek and smooth look also.

Get a rail no shorter then 12 inches to fully cover a mid-length gas system. I would say that a lot of "rifle" length rails would do the trick for you!

18in w/ a 13.2 Larue

20in w/ a 15in MI

As stated.. 12" minimum for good coverage. Midlength is 9" handguard typically.. Gas block is about 1"-2".  2" overlap is good. Midwest 12" works well and is reasonable as a railed FF.. Also, any of the 12" tubes (DPMS and Doublestar) work great and are really cheap (at brownells). I have helped friends and relatives install those and they have been happy.

Some like longer FF's but depends how much barrel you want to see. I like to show a little barrel. Also, the longer FF, the more weight. Although in this case, a 18" upper probably won't be dragged around much (unless used in hunting?)...

12" and up.  I like the look of the 15" rails on 18" barrels.

Troy 15"

The MI rail was a pain the the ass to get lined up.  The Larue was GTG from the beginning.