When I went to my local martial arts store, a woman who works there just looked at me from a distance and proclaimed that my keiko-gi (top) should be a size 3 and my hakama (pants) should be a size 5. She didn't measure me or anything, so I just took her word for it because a) my Japanese isn't good enough to know how to ask her for measurements, and b) the store doesn't have a fitting room.
I think both are a bit too big. The keiko-gi is supposed to be longer than the side-slits of the hakama. Mine is, but it's also little baggy. The hakama were about 2.5 inches too long -- they're supposed to sit at the waist and fall to the ankle. Until recently I just wore the hakama higher up around my ribs, but I finally got around to hemming them to the proper length.
The obi (sash) goes around the waist over the keiko-gi and under the hakama. When I was at the store they didn't seem to know what I was talking about when I asked for an obi, so a member of my Naginata club was really nice and gave me an extra one that she had. A Naginata obi is 1 foot wide; it's folded lengthwise into thirds similar to how a letter is folded to make it 4 inches wide. I don't know if obi have sizes; you just want it to be long enough to wrap twice around your waist and then tie in a bow.
Every keiko-gi that I've seen in person is white (though I've seen some videos of people wearing black ones). Naginata hakama are always navy blue or black; almost all the ones I've seen in person are black (including mine). My obi is white, but the 1st-year students at my base school got black ones.
I wear a tank top and yoga shorts under my gi (and undergarments under them). I don't know if those are the correct things to wear under a gi, but whenever I've worn them no one said I was doing anything wrong. Once I wore a bra (without a tank top) under my gi and when I was changing another member of my Naginata club asked if I was cold, which is the Japanese way of telling others that they're not wearing an appropriate amount of clothing.
Here's a rough list of Naginata gi sizes, based on size charts I've found online adjusted to the length of my gi. Disclaimer: It seems like different manufacturers and different countries have different sizing systems, so use these only as a starting point and try things on if you can. Also allow for shrinkage if your gi contains cotton and you'll be putting it in the dryer or ironing it.
Keiko-gi are measured by the length from the shoulder to the hem.
- size 1 is about 27.5 inches long (probably for someone 4'9" - 4'11" tall)
- size 1.5 is about 29 inches long (probably for someone 4'11" - 5'1" tall)
- size 2 is about 30.5 inches long (probably for someone 5'1" - 5'3" tall)
- size 2.5 is about 32 inches long (probably for someone 5'3" - 5'5" tall)
- size 3 is about 33.5 inches long (probably for someone 5'5" - 5'7" tall)
- size 3.5 is about 35 inches long (probably for someone 5'7" - 5'9" tall)
- size 4 is about 36.5 inches long (probably for someone 5'9" - 5'11" tall)
- size 4.5 is about 38 inches long (probably for someone 5'11" - 6'1" tall)
- size 5 is about 39.5 inches long (probably for someone 6'1" - 6'3" tall)
- size 1 (or size 22 in a different system) is about 29 inches long
- size 2 (or size 23 in a different system) is about 31.5 inches long
- size 3 (or size 24 in a different system) is about 34 inches long
- size 4 (or size 25 in a different system) is about 36.5 inches long
- size 5 (or size 26 in a different system) is about 39 inches long
- size 6 (or size 27 in a different system) is about 41.5 inches long
- size 7 (or size 28 in a different system) is about 44 inches long
- size 8 (or size 29 in a different system) is about 46.5 inches long
- size 9 (or size 30 in a different system) is about 49 inches long