I ordered a size 7 for my 5 year old, who usually wears a size 7 (sometimes a 6 if the brand runs big). It was difficult to say if the set was too big or too small, but basically, I had t take the vest in because it was VERY wide, but it was almost too short (he is tall, but I wouldn't say too tall for a 7). Luckily I only needed this for one day, 4 days after it arrived, so I had time to make alterations; I don't think he'll be able to use it 3 months from now, but it served it's purpose. The shorts fit him perfectly around the waist, but I had to let the hem down as much as possible because it was nearly mid-thigh. Ultimately, I'm glad I got the 7 because the 8 would have been too wide for the vest, and maybe for the shorts as well. The collared shirt underneath might fit him when he's 13. I used this for a baptism, so I paired it with a long sleeved shirt and never planned on using the one it came with. The tie is great. I ordered a white linen cap for this and it made a wonderful vintage inspired outfit.