FWIW, the E. parvula in my tank tops out at about three inches (or a bit under) so far. It's been in the tank for about two months, and is about 19" from my 2wpg fixtures. Fresh from the pots it was about 1.5 inches. Also, it is really slow to spread, at least so far in my tank (substrate related? (normal aquarium gravel)) so if you're looking for a quick carpet you may want to buy a lot.
You will find great confusion in the naming of the smaller hairgrass species. I can't tell you how many times I bought/wanted E. acicularis and gat parvulus or some other sub-species. Most suppliers seem to be unsure themselves sometimes.
The best place to get 'real' acicularis is at Lowes (havn't tried Home Depot). Unfortunately, they only sell it during warmer months when they offer their pond plants. It's labeled as needle spike rush. It is definitely much taller that the potted plants sold in fish stores. Eleocharis vivipara is the tallest hairgrass i've seen (also as seen in many of Amano's tanks as a background plant). I agree that many times the eleocharis acicularis sold in fish stores is really eleocharis parvula.
parvula or parvulus is shorter than acicularis. E. acicularis is very common in the USA because that is what Florida Aquatic Nurseries sells as dwarf hairgrass potted. I thought you guys bought from them Jeff! Parvula has to be imported. Parvula is the one most sought after and the hardest to find because it is imported and is the shortest variety. It is unlikely you would find parvula in pet stores unless they import. Vivipara is a horrible plant because the hair like leaves branch out all over the place growing into a thick spider like tangled WEB. Giant hairgrass is the tallest, Eleocharis montevidensis, which gets as tall as 2 feet and the leaves are about as big around as straw.
They are about the thickness of hair, possibly slightly thicker. I too have some of this 6" hairgrass. It was sold to me by a fellow hobbyist as acicularis. My true dwarf hairgrass (parvula) stays very short, around 2".