What is the relationship of type of motherboard and the size of ram

Each motherboard will say how much ram it can support.

The real relationship to maximum supported RAM is the CPU limits.

Examples:
LGA 1366 Bloomfield = 24GB {6X4GB}
LGA 1155 Sandy Bridge = 32 GB {4X8GB}

Some Server boards are 192GB~256GB+.

4CPUs 64GB/CPU:

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.

The Maximum is determined per CPU, and then limited by the MOBO.

LGA 1366 examples, 288GB -> http://www.supermicro.com/products/motherboard/Xeon1333...

i7 980X {24GB} Max - http://ark.intel.com/Product.aspx?id=47932
Xeon X5680 {288GB} Max - http://ark.intel.com/Product.aspx?id=47916