Closed transactions will be accessible in Method.
The Transaction History is how far back
Using your Sales Order example, if you were to change the transaction history from (for example) All to 365 days, Any Sales Orders which have been invoiced or manually closed that are older that 365 days old will get archived.
The transactions will still appear as they normally do in QuickBooks and as such, will not affect the size of your QuickBooks file. It will however make your Method File smaller, and should speed up the sync process.
Keep in mind that when you initially lower the Transaction History date, you will see a bunch of DeleteFromDesktop conflicts appear in Method for the archived transactions. After you've run the full sync a few times these should disappear.