what is the relationship of type of motherboard and the size of ram
Look at the motherboard specs.
Workstation motherboards may be able to support as much as 256gb of ram. More common P67 motherboards can support up to 32gb using 4 x 8gb sticks.
Some 1366 motherboards can support up to 48gb, again using 6 8gb sticks.
But... 8gb sticks are not yet available, so the real current limit is half.