You do not have high enough ceilings to do larger than 5.25 crown molding it appears. The reason size is very important can be demonstrated with this example. I had an old neighbor with a new custom built home. Her husband installed their crown and baseboards. Mary always wanted more, bigger, better...even though she didn't have any more than 9 foot ceilings. So they installed too large crown and baseboards to make Mary happy. Nothing complex, just very oversized for the space. It looked ridiculous, unfortunately. Due to the white oversized trim top and bottom, the walls looked shorter than ever. In effect, they shrunk the room! Also remember to install crown that corresponds to the baseboard. You can even go to the big box stores to get advice. If you hire someone to do the job, they will know the correct size. Good luck, hope it looks fabulous for you.

Here is a link that might be useful: crown molding size chart