Does anyone know what the white stuff that's building up around the suction and liquid line on the supply plenum above the furnace is?

This is a picture of what Iam talking about

That is ICE! Probably caused by a low refrigerant charge or lack of air flow from a dirty filter. Check to see if the "supply plenum is insulated also, this freezing happens when the ambient temp dew point can freeze when evaporator coil is so cold.

that is ICE !! probably caused by a low refrigerant charge or lack of air flow from a dirty filter. check to see if the "supply plenum is insulated also, this freezing happens when the ambient temp dew point can freeze when evaporator coil is so cold.

Thanks. Can you help me with another question?

How would I identify: 1) The type of system
: 2) Its size from the picture below.

Looks to be a 2 ton R22 system. Check your filter, and even remove a lower portion of sheet metal (directly on top of the furnace) so you can look up and see if the evap coil is plugged with dirt/dust etc. Make sure the coil is completely thawed when doing this, or you won't be able to tell. If that all looks clean, you will need to get a tech in to check charge and possibly locate and repair any leaks in the system. As heatactech said, 99 percent of the time a freezing coil is due to lack of airflow (could be dirty filter, plugged evap coil, extremely dirty blower wheel, weak blower motor/bad motor capacitor, plugged secondary heat exchanger if furnace is a 90+ eff unit, or lack of or closed off supply or return grills) or low charge of refrigerant.