rony1992,
There are no hard and fast rules, so just about anything you want to weld to it, provided you observe the basic dimensions shown in API6A. You can transition up or down in size, if you comply with the requirements shown for Welding Neck Flanges.
Materials are another matter. Materials requirements tend to vary from application to application. What pressure/temperature class will you be working with? Which piping material is specified? What does your weld procedure require?