Up-potting go by root size v.s. leaf size?

[Meg25]

I think it is time to repot this noid phal. seedling, it's roots were growing out of it's 1 1/2 inch pot. But I am not sure how big of a pot I should put it in. This is a pic of the plant with the pot I am considering, it is 3 1/2 inches, and the roots would fit in it nicely.

The leaves however are only about 5 inches across.

I plan on putting it in sphag. moss. Not sure if that will make a difference.

Any suggestions are welcomed!