You could change out all the window trim to flat trim and then run the crown moulding over the flat trim, bumping it out around the top of the window.

How large is your crown going to be? Will it be much larger than the trim around the top of the window? If possible, I'd try to incorporate it so it blends into the crown. Like the third and last picture at this link: http://ttecustomhomes.com/portfolio-2/ceilings/

If there isn't enough room to do that, then you might just have to end the crown on either side of the window. I think it's called a return.

If you are hanging window treatments, you might also build out a valance over the window that integrates with the crown. I couldn't find a link directly to a picture, but on this page there is one about half way down on the right. http://www.traditionalproductreports.com/woodwork-historic.html

As TSG said two of the options are to incorporate the transition, or hide it. It is hard to know which will work best in your space without knowing how big the crown will be and what style you have in mind, etc.

Here are some examples of incorporating the window trim with the crown:

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[Traditional Bathroom[(https://www.houzz.com/photos/traditional-bathroom-ideas-phbr1-bp~t_712~s_2107) by Stamford Architects & Designers Sound Beach Partners LLC

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[Traditional Bathroom[(https://www.houzz.com/photos/traditional-bathroom-ideas-phbr1-bp~t_712~s_2107) by Norwich Architects & Designers Smith & Vansant Architects PC

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[Traditional Bathroom[(https://www.houzz.com/photos/traditional-bathroom-ideas-phbr1-bp~t_712~s_2107) by Boston Kitchen & Bath Designers Venegas and Company

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[Contemporary Bathroom[(https://www.houzz.com/photos/contemporary-bathroom-ideas-phbr1-bp~t_712~s_2103) by Bethesda Design-build Firms Case Design/Remodeling, Inc.

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[Traditional Bathroom[(https://www.houzz.com/photos/traditional-bathroom-ideas-phbr1-bp~t_712~s_2107) by Boston Architects & Designers Catalano Architects

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[Traditional Bathroom[(https://www.houzz.com/photos/traditional-bathroom-ideas-phbr1-bp~t_712~s_2107) by Canton Furniture and Accessories The Furniture Guild

Or transition with window treatments:

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[Traditional Bathroom[(https://www.houzz.com/photos/traditional-bathroom-ideas-phbr1-bp~t_712~s_2107) by Los Angeles Interior Designers & Decorators Just Joh

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[Traditional Bathroom[(https://www.houzz.com/photos/traditional-bathroom-ideas-phbr1-bp~t_712~s_2107) by Chester General Contractors ColeBuilt

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[Traditional Bathroom[(https://www.houzz.com/photos/traditional-bathroom-ideas-phbr1-bp~t_712~s_2107) by Detroit Interior Designers & Decorators Urso Designs

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[Eclectic Bathroom[(https://www.houzz.com/photos/eclectic-bathroom-ideas-phbr1-bp~t_712~s_2104)

and this one, sort of in between:

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[Traditional Bathroom[(https://www.houzz.com/photos/traditional-bathroom-ideas-phbr1-bp~t_712~s_2107) by Charleston Architects & Designers Frederick + Frederick Architects

I hope this gives you some ideas!

I haven't picked yet cause I was waiting for the feedback. But I was thinking 3 1/4 or 4 1/4 and not too elaborate in terms of ridges and steps. GC said he could cut a slit and go over the window trim. I am just not sure if it would look good. Or maybe better completely replace window trim to a more basic one. Or a thinner one so there is space for crown above it. But the window is pretty high.

Thanks