Choosing a Jita for you companion plant, kusamono or figurine is very much personal choice as with many display items, however a good way to decide is to look at how professional Bonsai people display their items. Exhibition books are probably the best way to do this unless you travel to exhibitions a lot!
Every companion or kusamono needs a display stand or jita. The size and style matter in that something too obvious may very well take away the focus from the items you are trying to highlight in the display.
The very simplest stand or jita for a companion or kusamono is one that is just visible, like the photo example here.
This depends on the overall display design. You may want a jita or stand that makes the accessory more obvious for a particular reason, so maybe a larger jita or higher stand, might be more suitable.