Here is a generic gi size chart for Judo / Jiu-jitsu gi. Pick a gi based on your height, and you should be good to go. For example, if you are 5'11" and weigh 180 lbs, your size is 4 by height, but 5 by weight. The size 4 will fit you little snug, and size 5 will be little roomy. Wash in cold water and drip dry only. Just in case you picked a wrong size, just ask for exchange. No big deal. Because you washed it once, technically it's not "brand new" but it's not exactly used either. Just don't wear it past a thanksgiving dinner with gravy sauce and say the gi is too small.
1. Wash your gi as soon as possible after each practice.
2. To prolong the life of your gi (and to prevent shrinking), wash in cold water and hang dry.
3. Avoid using bleach or fabric softeners, which may damage the material.
4. Oxiclean (used sparingly) may help remove stains. A cup of baking soda in your wash will help remove odors and keep the gi white. Adding a cup of white vinegar will help kill mold.
5. Interestingly, some judo sites recommend using salt and different combinations of baking soda, vinegar, and citrus juice to remove stubborn stains. I haven't tried this myself, so I'm wondering how well it works.