Increasing the speed of an object when the scale is decreased and vice versa

I am working on a game which involves cells floating in world space. The player will start off with a size of 100 and a speed of 10. I just need some sort of formula which will decrease the speed of the cell as the size increases. I can't seem to figure it out, maths was never my strong point. So for example when the player hits 150 in size their speed may be around 8. I know I could do this manually by checking if the size is above a certain amount and setting the speed accordingly but this formula would make it a lot smoother and more efficient in general. Any help is greatly appreciated, thank you!