The ideal size of a curling iron depends upon the need of the user, so there is no one size that is ideal especially when using on a professional level.. However, you can use the curling iron depending on the desired results.
For creating Retro Look
In order to create an ideal retro look a ½ inch ceramic tourmaline curling iron is the best. If 1-inch hair sections are parted and curled outwards it can give you gorgeous retro look and you may get back to styles they flaunted back in the early 20th century.
For touching up
If you already have natural curls but still need to look balanced and voluminous then using ¾ inch curling iron is the best solution.
However, a 1-inch curling iron is considered as the safest choice because this will give great rolls and curls. This type of curling iron gives tight curls that last the longest.
For medium to long hair
Using 1 ¼ inch barrel size curling iron will give you big voluminous waves, adding tins of volume and free-spirited personality is reflected after you do the hair setting.
Jumbo blown-out ends
Using a 2-inch curling iron means knotting huge sections of hair that give a great blow out on the head and a kick to the end of the hair, It is great to do on a professional level to create exquisite looks.