My daughter was one of those kids who was "deceptive". She wore a size 5 right as she turned 3 years old (she's 6'2" now). She was tall but skinny. Size 5 for length in store bought clothing, size 2 or 3 in width. Definitely get measurements and measure the pattern pieces before you cut. Not all patterns, designers, etc. were created equal. Some had more ease than others. Tailoring ahead of time saves a lot of grief later.