The ideal size - for me, anyway - is about the size of a massage table, but slightly higher.
An adult-sized changing pad is very difficult to find, but you can probably make one or have it made. Something like a yoga mat makes a good foundation; you’ll just want something thicker or more substantial.
The area beneath the changing pad is very handy storage, whether built out with drawers, shelves, etc.
Of greater concern than your comfort is the comfort of the person who’s changing you. The changing surface needs to be at a comfortable height, so a little trial and error in the design process will help you identify the correct level. You can try other tables - dining table, office desk, etc. - to find a height that ‘feels’ right. In my experience, anything above about 30” tall will present some problems with reach or access.