Before you decide on the size of the pot, you need to check the roots. If the den has been overpotted for the past 3 years, it may have lost many of its roots to rot. If that's the case, a 3" pot would be sufficient for it until it grows a new root system. It won't hurt the plant to go bare root for a day, so you should unpot it, and check the roots by gently pressing them with your fingers. If they're firm, regardless of colour, they're still good. If they're soft and mushy, and the outer layer slips away leaving a thin thread behind, they're rotten and useless. Don't be surprised if it has very few viable roots.

Once you've chosen a pot for it and repotted it, make sure you secure it in the pot. Lots of people like bamboo stakes; personally, I prefer wire clips. I buy 16 gauge galvanized steel wire (any hardware or big box store, costs a couple of $$ for a lifetime supply) and bend it into clips using pliers. Sometimes a simple upside down U shape, going from one side of the pot across to the other, is enough to secure a plant. I have two big cattleyas whose root systems were killed by tightly packed sphagnum held in place with such upside down U's. If you're using a soft plastic pot, however, you may have to use a stake anyways because a clip may damage or at least distort the pot.