When the magma rises (from the mantle) at the spreading ridges and then cools, what happens to the size of the plates?

Increases

Explanation:

The spreading of ridges is a divergent boundary. The oceanic crust diverge or move away from each other as magmatic material migrate from the mantle to the earth crust through the mid oceanic ridges and this later forms a new crust.

Divergent boundaries which are mostly prevalent in the oceanic crust involves the spreading away of the sea floor. The sea floor moves apart giving way for the up-welling of magma from the mantle region.The magma is a semi liquid, as it moves to the earth surface it began to cool and this semi liquid later solidifies to form a new crust. The new crust are formed at the mid oceanic ridges. As one move away from the mid oceanic ridge the rocks becomes older.

The new crust formed have been added to the formal size of the plate, that means their is an increase of the size of the plate.