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you will find the Image class:
http://www.itk.org/Doxygen/html/classitk_1_1Image.html

A ctrl+f for "RegionType" will show you that it is a typedef.
Following the links you will get to:
http://www.itk.org/Doxygen/html/classitk_1_1Region.html

It is actually hard to follow from there, because the typedefs are
actually of the Superclass, so you might have had to go to ImageBase
instead? But the better answer is just to use

itk::Image<DCMPixelType,> 3>::IndexType

directly, as it is the same thing. RegionType is actually an
ImageRegion http://www.itk.org/Doxygen/html/classitk_1_1ImageRegion.html
, for which you can find the SetSize function. (SizeType is an
itk::Size).

Good luck,

David