High Grade kits have two scales: 1/144 which is around 13 cm tall depending on the model. Some models are a little bit taller like Sinanju and Sazabi, while some HG models are somewhat shorter like Exia. 1/144 kits are nice to collect and display because they occupy less space than Master Grade or Perfect Grade kits. The other scale is 1/100 which is around 18cm tall.
You can readily determine that it’s a High Grade kit because it will have the abbreviations “HG” on the box and its box art is in landscape format. As for the 1/100 Scale High Grade kits, you won’t be seeing any HG letters on the box, though you’ll know it’s a 1/100 HG kit if it doesn’t have “MG” or “Master Grade” printed on the box art, and the box art is usually in portrait format. Don’t get confused between 1/100 HG and 1/100 MG kits! First timers might get confused because they looks the same.