If i remember well, you load a single image file into the TDXImageList and set it on the TDXFont. To achive what you want i think you should set the tile size (pattern width, pattern height) to the biggest char on your image.

For example, you create an image with tiles of 48x48 and then you can have a character of 10 pixel, 12, 16, 20, 24, 32, 40, whatever.
The problem is that spacing between character would be different depending on the char size. To solve this i'm afraid you should create your own TDXFont component that inherits from it and handles the space between characters.

Let's see if someone else has another solution