Here's a quick, rough sketch to give an idea of what measurements I will need from someone when making a costume for them.
All measurements should be taken while standing with feet shoulder width apart (note that the figure in my sketch is standing with its legs a little too far apart). The arm length should be taken with the arm at your side, elbow bent. If at all possible, get someone to assist you. Measurements shown with arrows > > indicate that the measurement should go all the way around the body and meet back at the starting point.
Measurement key:
1: Height
2: Shoulder to Ankle
3: Neck
4: Chest
5: Waist (natural waist, not necessarily where you wear your pants.)
6: Hip (over fullest part of your buns and hips)
7: Inseam (crotch to ankle)
8: Wrist