The Dollmaker has particular requirements if you want your sprites to work. View the existing parts (and palette and image size) for specific doll sets here
First and foremost: COLORS ARE VERY IMPORTANT. Do not use colors outside of this pallete unless you want them to be unreplaced.
All KID/TROLL/CHERUB/SATYR parts are 400x300 pixels. YES even if it's something small like eyes. There will be a lot of empty space, this is normal. For any other doll set check the test page to get an idea of sizes.
If you want something to be part of a layer, position it exactly where existing things are on that layer. Put your eyes where the eyes already are, for example.
Things like hair and fins LOOK like a single layer but are actually multiple. Part of the hair/fin goes on top of the face, part behind.
Here are example images for all layers. You can save these pictures and use them as guidelines for aligning parts.