I just bought a Grammatocymbidium (l like typing that whole name) Mem. Nathan Newman 'NN'. It's pretty much filling the pot that it's in and the newest p-bulb is actually straddling the outer lip of the pot so it needs to be repotted. I like clear pots with slots or holes for air circulation and visibility to the roots media, so I'll probably end up having to order pots of the 'Net.

So how do i determine how big the existing pot is and how big I need to go with a new pot?

I've got some pots that are too small, but they have a similar structure to lots of pots, in that they taper towards the bottom and have an area at the top that's a bit bigger than the rest and then a lip at the very top. At the very bottom, the outer diameter is about 3.25". Near the top, it's 4.25", and the widest part of the outside lip of the very top is 4.5".

The pot that the Nathan Newman is in ranges from ~4.25" up to ~4.75".

I'm guessing that I'd need a 6" pot for that guy.