Invert 2 values

[Tornado77]

Hi, sorry if this is a dumb question, but my background in mathematics is limited (I'm still at high school). I need to "invert" 2 values. I'm trying to create accurate stars for a game, to change their size I'm using the distance from the earth, the magnitude of the star, and the class. The problem is that the result is the reverse than what I want. Sirius, the brightest/one of the bigger stars in the sky is dozens of times smaller than HIP 15714, a random star which is(in real life) way smaller and dimmer. What I need to do is to invert them, so Sirius is the biggest and hip is the smallest. How can I do that? (Sorry for bad english)
This is a screenshot from Stellarium just to show the difference between their sizes.

 

[Dr.Peterson]

I don't understand. How are you finding Sirius to be smaller? Maybe you need to show us what you are doing specifically, so we can see your error.

But possibly your assumptions are wrong.

For the most part, we can't see the size of a star at all; if one looks bigger to us, that is an illusion caused by its greater brightness. (See https://en.wikipedia.org/wiki/Magnitude_(astronomy), "To the unaided eye, a more prominent star such as Sirius or Arcturus appears larger than a less prominent star such as Mizar, which in turn appears larger than a truly faint star such as Alcor.")

And the (apparent) magnitude of a star (which refers to its relative brightness as seen from earth) already takes its distance into account. I am assuming you are using apparent magnitude, not absolute magnitude, because it is only in the former sense that Sirius is the brightest.