Magnetoplasmadynamic thrusters are an interesting proposed method for propulsion in space, but are there any upcoming programs that will them? If not, might this be because there still serious and unsolved technical challenges?

Also, roughly speaking, how big might these the thruster be?

From Wikipedia's Magnetoplasmadynamic thruster:

A magnetoplasmadynamic (MPD) thruster (MPDT) is a form of electrically powered spacecraft propulsion which uses the Lorentz force (the force on a charged particle by an electromagnetic field) to generate thrust. It is sometimes referred to as Lorentz Force Accelerator (LFA) or (mostly in Japan) MPD arcjet.

[...] According to Edgar Choueiri magnetoplasmadynamic thrusters have input power 100–500 kilowatts, exhaust velocity 15–60 kilometers per second, thrust 2.5–25 newtons and efficiency 40–60 percent.