Doors typically come in a range of popular standard sizes and can often be trimmed a little to get an exact fit. This is true for kitchen doors, bathroom doors, bedroom doors, and all doors found in most homes. Custom doors are available which can be made to measure for doorways which do not match the standard sizes, but these will be more expensive.
Internal door leaves are typically 35mm (1 3/8") or 40mm (1 2/3") thick. You can also get 44mm (1 3/4") and 54mm (2 1/8") doors - these thicker ones are usually fire-rated doors. Below are common standard door leaf sizes; individual manufacturers may not carry all these sizes and some might have a larger range of standard sizes.