I am going to install hardwood flooring in my family room and hallway. I found a good deal on 3/8 X 3" hardwood but have read that anything over 2 1/4" wide is to be used in large rooms only. My family room is approx. 290 sf (16X18).....
The hallway is about 4 feet wide by 12 feet long......
I have the space carpeted now... it is on the main floor of the home with a plywood subfloor.....
Any advice?

The width of a floor board in relation to the width of the space it is in is mostly a matter of personal esthetics.
If you have found a great deal on a product, and it is of adequate quality, dont dwell too much on what you read somewhere about "only using boards wider than 2 1/4 inch in big spaces". A 16 ft wide room can easily accomodate even 5 inch or 6 inch planks. Same with the 4 ft wide hallway.

Yeah, I never heard of such a rule. Around here, a popular floor in the 60's and 70's and into the 80's was 3-5-7 width random plank. We also see the same product in a 4-6-8. It went down hallways, in closets, in bedrooms, etc. It always looked fine to me.

Thanks guys for the help..... I have a few more questions....
The hardwood I'm looking at is 3" wide by 3/8 thick.... I see a lot of people go with 3/4 thick.... Will the 3/8 thickness be a problem?
The area I'm putting this hardwood on has wall to wallcarpet and pad at the moment.....

Finally, what type of underlayment do I want to get? There is the felt paper; poly sheeting; the dream home insulament; etc.....
Thanks

Is the 3/8th inch product solid or engineered? If engineered, what is the thickness of the wear layer? re-sandable? if not, take care of the finish or you will be replacing the floor in a few years. If solid, I would have some concerns about stability, unless you control the home environment to maintain a fairly uniform humidity level during seasonal changes.
Otherwise, dont worry too much about it. 3/8th strip flooring was the norm up here until the advent of pre-finished products, with 3/4 inch being a very big upgrade.

In the states, the normal unlayment for hardwood seems to be 15 lb felt, here in Canada, except Quebec, we just use wax saturated paper, and it works just fine.

Can you post a picture of it Dennis ? Aqua Bar B is also very good . A little more money though .
Seems like consumers are moving away from 2 1/4 here and going with the wider board .

It is a 3" wide 3/8" thick solid hardwood --- It's from Bellawood.....

What about this statement guys?? ----Is this true??
"When a strip floor is too thin for it's width it will have a tendency to warp, when great variances in indoor relative humidity occur. For instance a 1/2" thick strip that is 3" wide is a very poor choice, but a 1 1/2" by 1/2" strip should be quite stable. So, as a general rule, try not to exceed a 4 to 1 ratio of the width vs. depth, and 3 to 1 would be ideal. That makes the 2 1/4" by 3/4" strip floor the most stable and longest lasting hardwood floor." --- Now I'm worried --- Is my 3" wide by 3/8" thick flooring a good idea? It is Brazillian Koa and solid hardwood.
Thoughts?

Well, I will post a pic of the underlay we use here, but essentially, its just like butcher's paper, and cheap as Heck. 750 ft roll @ 11.00 bucks CAD.
Probably why we use it, were known for ummmm frugality.
Floating floors of course, we use the proper underlay but general school of thought is the paper for nail down applications is more to assist the installation rather than act as a vapour barrier.
In Quebec, they use the same 15 lb felt that you guys use down there.

I agree with the statement you just posted, Timmy; except that if the environment is properly controled, the risks are reduced significantly.
Since you are going with an exotic wood, consider the environment it evolved in, and the environment you are placing it in.

when great variances in indoor relative humidity occur all wood is going to react .

“As stated in NWFA Tech manual , Pg 4
Wood is hygroscopic -- Meaning when exposed to air. Wood will lose or gain moisture
Until it is in equilibrium with the RH and temperature of the air”

Exactly, which is why I am constantly reminding the customer to maintain stable interior temp and humidity levels. Most ignore me and wonder why they have seasonal gaps.

Don't feel bad Gary , I just had a customer tell me he didn't need at least 6 mil Polly over his dirt crawlspace because it is dry under there .
And i went and wasted all this money on NWFA installation and Inspectors classes when all i needed to do was ask him .

I am constantly amazed by customers, especially general contractors, who question why I recommend certain procedures. Or who basically state that, although they have never laid a floor, know how it should be done! Really? Then why hire an expert? Simply get that immigrant out in front of Home Depot and tell him what to do. No, what they want is an experienced licensed pro they can try to boss around. My current customer is constantly bugging me over things that are not problems, yet he "feels" that they are.
Example: He buys an unfinished, handscraped walnut plank floor with handscraped bevels on all sides. Then he wonders why the bevels are not perfectly uniform. Another example: The distributor told him the flooring needs to nailed and glued. OK, fine. But he tries to insist it must be a full spread job. I explained that 1) Sika makes an adhesive in a cartridge form because it can be used to glue down solids. DUH! 2) The distributor is trying to sell him three times as much adhesives as he needs 3) I still need to screen and finish this floor. Full spread is WAY more messy than cartridge applied 4) The cartridge method puts out LARGER beads of adhesive than troweling 5) The floor is being nailed as well. Easily doubling the holding power. So, do you think he listened to me? Not really. Anytime he hears the slightest noise, he thinks that with a full spread job, there would never have been any movement.

Some days, I tell ya. I told him he could get second opinions but he hired me and I'm doing it the way I think it should be done.

Its alright gary, just make sure you lay the flooring following the "sun" rule, forget about crossing the joists