Why my 3D-printed model doesn't match the real size?

I remodel a human heart, actually I used my Mom’s CT Angiography data, and after segmentation, I exported a stl file and 3D-Print it.
But the model is so big and doesn’t match the real size.
I was wondering what should be done before exporting the stl file to have the real size?
Your prompt response is highly appreciated.

The STL files that Slicer creates from DICOM CT files uses millimeter as unit by default. You can change that by specifying a custom “Size scale” in the “Export segments to files” window:

If you want the file to contain coordinate values in meter then set size scale to 0.001.

Make sure that you chose the corresponding unit in your 3D printing software.