When you're staring down the barrel of a new diet, your portions are one of the most difficult things to measure and keep track of. Your kitchen scale may be great in the comfort of your home, but it's not practical to carry with you all day. Instead, just get to know the rough estimates with your hand.
1 cup = your fist
1 ounce = the meaty part of your thumb
1 tablespoon = your thumb, minus the meaty part
1 teaspoon = the tip of your index finger
1 inch = the middle section of your index finger
1-2 ounces of a food like nuts or pretzels = your cupped hand
3 ounces of meat, fish, or poultry = the palm of your hand
Obviously these are rough estimates, and their accuracy will vary based on the size of your hand, but it's not a bad starting point. If you happen to have a kitchen scale at home, you could use that and some measuring spoons to determine just how well these measurements work with your hand and set your own baselines if any of the measurements are considerably off.