Commish Question - Dynasty League Rules

[BeefSandwiches]

I have played in re-draft and keeper leagues, but never a dynasty. What is the typical roster composition and is there a requirement for how many players are kept from year-to-year? Do you have "practice squads"? For the rookie/free-agent draft, is there a roster limit or do you simply use a cutdown day prior to the start of the season?

I know this is pretty general but any help will be appreciated.