Size is essentially unlimited, maybe a few hundred thousand words.
As well, you can define multiple dictionaries. I do this because I write in several very different domains, where common jargon or acronyms in one domain would be spelling errors in another. All this is controlled by:
Start Word
Select Tools | Options from the menu.
Click the “Spelling and Grammar” tab.
Click the “Dictionaries …” button.
From there you can add new dictionaries, edit them, or remove them. This is where you’d give your new Word installation a copy of your old dictionary.
You can also have multiple dictionaries to deal with multiple languages. Good English spelling is lousy French spelling and vice versa.