Looks:

Gender:
So the most obvious question is that of gender. Do you want a male or a female character? Many confuse physical gender with what gender the character feels. I tend to list it out in bios (the ones where it's relevant anyways) as follows:
Gender: (physical)
Sex: (Sexual orientation, preference, etc)

Example: Ma'at's gender is female, but she has a bi orientation.


Height and Weight Ratios
A lot of fan creators aren't aware of the proper height/weight ratio and thus wind up with extremely anorexic or overweight senshi. Thus I give you the following charts from Information Corner.

Weight is given in lbs

Height -Weight Table for Women (Weight in pounds)

Height
(Feet Inches)
Small
Frame
Medium
Frame
Large
Frame
4' 10" 102-111 109-121 118-131
4' 11" 103-113 111-123 120-134
5' 0" 104-115 113-126 122-137
5' 1" 106-118 115-129 125-140
5' 2" 108-121 118-132 128-143
5' 3" 111-124 121-135 131-147
5' 4" 114-127 124-138 134-151
5' 5" 117-130 127-141 137-155
5' 6" 120-133 130-144 140-159
5' 7" 123-136 133-147 143-163
5' 8" 126-139 136-150 146-167
5' 9" 129-142 139-153 149-170
5' 10" 132-145 142-156 152-173
5' 11" 135-148 145-159 155-176
6' 0" 138-151 148-162 158-179


Height -Weight Table for Men (Weight in pounds)

Height
(Feet Inches)
Small
Frame
Medium
Frame
Large
Frame
5' 2" 128-134 131-141 138-150
5' 3" 130-136 133-143 140-153
5'' 4" 132-138 135-145 142-156
5' 5" 134-140 137-148 144-160
5' 6" 136-142 139-151 146-164
5' 7" 138-145 142-154 149-168
5' 8" 140-148 145-157 152-172
5' 9" 142-151 148-160 155-176
5' 10" 144-154 151-163 158-180
5' 11" 146-157 154-166 161-184
6' 0" 149-160 157-170 164-188
6' 1" 152-164 160-174 168-192
6' 2" 155-168 164-178 172-197
6' 3" 158-172 167-182 176-202
6' 4" 162-176 171-187 181-207


Some points of reference here:

Sailormoon: 4'1''
Sailormercury: 5'2''
Sailormars: 5'3''
Sailorjupiter: 5'6''
Sailorvenus: 5'
Sailorsaturn: 3'9''
Sailoruranus: 5'8''
Sailorneptune: 5'4''
Sailorpluto: 5'6''
Tuxedo Kamen: 6'2''

Build:
This is often neglected. Telling someone the height and weight of a person doesn't help. Generally, there are three categories of body type to define for your character. The first category has skinny, plump, and slender, defining general shape. The second focuses on their appearance: delicate and fragile, tough and wiry, or curvaceous and sensual. The third defines physical fitness: heavily muscled, well toned muscles, average strength or physically weak.
Here are some examples of what the senshi are like:

Sailormoon: slender, delicate and fragile, average strength
Sailormercury: skinny, delicate and fragile, well toned muscles
Sailormars: slender, curvaceous and sensual, average strength
Sailorjupiter: slender, tough and wiry, well toned muscles
Sailorvenus: plump, curvaceous and sensual, well toned muscles
Sailorsaturn: skinny, delicate and fragile, physically weak
Sailoruranus: slender, tough and wiry, heavily muscled
Sailorneptune: slender, curvaceous and sensual, average strength
Sailorpluto: slender, tough and wiry, average strength

Hair and Skin

You also see a lot of characters with very boring hiar and skin tones. Here are some charts that may help you out, especially if you're doing your senshi images on the computer or requesting from a digital artist. RetouchPRO color charts has helped me out in creating more unique senshi looks. Bunch of browns is just what it says as is Shin Tones tutorial.

Clothing



People also tend to leave out what kinds of clothing their character likes to wear. While in the anime and manga, we tend to see the characters in their school uniforms fairly often, they're not always going to school. It can be helpful to potential fan artists to know what kinds of things they like to wear out side of the class room. This section can also be used to describe their school uniform (if they have one) if it's not one of the ones you see in the show/manga.

Example:
Physical Appearance: Kougi is very tall, standing at 5’11 with long, thick teal hair and dark green eyes. Her skin tone is average, but she has a mole under her left eye. She hates it and tries to cover it up with makeup. She has a longer neck than normal, a fact she tries to hide by wearing high collared shirts and turtle necks. Her hair is always up in a very poofy ponytail.
Clothing Style: She never wears skirts and dresses if she can help it. Her preference is wear loose pants and long sleeved shirts. In hotter weather, she'll wear shorter sleeved ones to keep herself from over heating. Kougi was born with a sight impediment, so she wears thick glasses. She's rarely seen reading outside of her home because she hates wearing the stupid things.