The measurement for the miter return is equivalent to the projection of the molding, this will depend on what molding you want to use. To gain space, you can use side casings that are less wide. You can figure all of this by drawing it out on something to full size. Changing the angle won't return 90 degrees in a straight miter cut. You can use the sum of 90 to turn a corner but it gives a different look without much space savings but maybe enough in your case.
In normal houses, there isn't a lot of additional space to the sides of doors unless it has been planned for wide trim and that often means wider hallways and doors moved out from corners as they go into rooms.
When drawing the full scale layout, start with the available space and work backwards from that. The mitered returns on the top head casing have to fit in that space. Work from that back to the width of the side casings. The larger the crown molding, the more projection. You can still get the look with smaller crown, which will save space.
Here are some options for cutting mitered returns, the 22.5 degree cuts on the green molding will save a little compared to a 45.