Follow a few rules of thumb to keep Web content clear and concise:

MS Word has a reasonable word count facility, or try something like AnyCount or AnyMini W if you're writing directly in HTML (just remember to keep a word count of navigation link text, copyright text etc. and deduct the total from the word count for the page).

There'll be exceptions to all of these rules, especially for page length. That's okay, people will scroll if the information is what they need, and the scroll wheel mouse has made scrolling less intrusive.

If it has to be one long, single page, then it has to be - just make sure it does have to be, and make every single word on the page earn its keep.

Edit, re-write and restructure. Then do it again. There's a good chance you'll be able to keep to the maximum word counts after all.

Scannability and Readability in Web Writing What is Metadata in Web Writing?

Related:
Clarity and Brevity in Web Writing
Counting Web Words in Microsoft Word
Using MS Word Readability Statistics for Web Writing