Jenny is right a wood usually covers a relatively small or confined area whilst a forest covers a large area in some cases thousands or even millions ( the Amazon) of square kilometres.

Here in the Mid United States, we use "forest" to indicate acres of trees growing wild somewhere. "I spent five hours walking through the forest today.". When we think of "wood", we think of material we cut from a tree to burn in a fireplace, or to use to build a house. "I bought some wood at the store to build my shed.". Additionally, we often call the forest "the woods". "I spent five hours walking through the woods today.". Yes, english can be confusing, can't it?

the "wood" is smaller than "forest"