I'm not sure you're using "order" the way it's usually used. The order of a B+ Tree refers to the maximum number of children an internal node could have. Can you be more specific as to what you're asking?
Are you asking for a way to determine what is the optimum order for a given B+ Tree? If you think about it, you will see that this "optimum" value depends on the cost to do these things:
Follow a link to a child node
Find the appropriate key within a node
Insert a key into a node
Re-balance by splitting
Remove a key from a node
Re-balance by joining
Of course, these costs vary with platform, with underlying storage, with the size of the cache... And the importance of each also varies: How often do you do inserts or deletes compared with how often you do a lookup? This is a fun problem that can be approached in several ways.
In theory, you can just talk about Big Theta costs (though you will have to think hard about the meaning of 'N')
In a lab, you can use test data to measure the effect of changes
For a paper, you can do research to see what the industry does
For a thesis, all of the above. And more. And more...
Donald Knuth's volume on Sorting and Searching has some superb explanations of this stuff. Donald Knuth, The Art of Computer Programming. I think S&S is volume 3, but since I am on vacation this weekend, I'm not at home to look at the book and tell you precisely.