Shrinkage means, when the fabric becomes smaller than its original size, usually after washing. These days, novice users many times face accidental shrinkage of garments after washing, especially after heat. Most of the modern fabrics undergo a finishing process known as sanforization. This is the method of shrinking, stretching and fixing the woven cloth in both width and length before usage of the cloth to avoid shrinkage, which would occur otherwise after washing.
If you have bought custom dress shirt and it is not pre-washed or pre-treated then it will probably be cut accordingly to accommodate the shrinkage. The shirt is typically shrunk after very first wash. The intensity of shrinkage depends on the nature of the fabric used. In high quality custom dress shirts, the shrinkage takes place evenly, which means no part will shrink more or less than the rest. However, most of the custom dress shirts' manufacturers pre-wash the fabric to avoid the hassle faced by user.
Most of the synthetic fibers used to produce wrinkle free clothing are treated with chemicals that should not get in contact with your skin. Chemicals like formaldehyde, perfluorinated or PFCs with Teflon and sulphuric acid are highly dangerous for human skin. These chemicals, even in tiny amounts can cause headaches, breathing problems, skin and eye irritation.
In order to avoid shrinkage at all or you think that the shirt fabric have chances of shrinkage then do read the tags inside the shirt. The tags are very important to read, especially before using or washing for first time. Some cotton fabric shirts have special instructions of using no hot or warm water. Using hot water while washing or using high heat while drying is not suitable for first wash, which is very true for cotton based fabrics.
Wash your new custom made shirt in cold or normal water and try to dry it without the help of dryer, if you insist on using dryer then set it on lowest heat. Even after many washes, it is better not to use high heat while washing or drying. Hot or warm water is basically needed when the clothing is extremely dirty. The best way to fully clean your custom dress shirt is to first use stain remover and removes all the dirty or solid stains (read how to remove stains). When stains are removed then you can wash your shirt easily using luke warm water.
There are some people, who try to intentionally shrink the shirt for tight fitting but they never get the desired results. If you have too large shirt then take it to the tailor instead and resize your shirt according to your needs.