What version are you currently on. There is no migration involved unless you're currently on version 6 or 7. For any other versions, there are some in-place SQL updates to the databases.
Depending on what the current version is, it might be necessary to re-create the search catalogs for your repositories. That can be done while users are working in the repository, just be aware that search results may be incomplete while the search catalog is being repopulated.
If you're not moving servers as part of the upgrade process, the total repository size on disk is not really relevant. The number of entries or the field values may be relevant. After installing the new version, when the Laserfiche Server starts up it will detect the version of the repository (by reading the version value from the dboptions table) and will run SQL scripts to get it from its current version to the version matching the Laserfiche Server. You can test out this process by restoring a copy of the SQL database as a separate database on your SQL server and manually running the scripts (they're located in <LFServer's install folder>\sql and indicate the version they upgrade from in the comment at the top - match the version in your dboptions table and go from there in order until you get to the latest. You don't have to install the newest version, you can just extract the files from the MSI to get the SQL scripts). That should give you an idea how long updating each database would take when the Laserfiche server does it.
Workflow and Forms will also update their databases on upgrade, but not to the same extent the Laserfiche Server might, it's just adding new tables and stored procedures, not modifying existing entries.
As for how you might go about it, the order is usually: LF Directory Server, LF Server, Forms, Workflow, then the client applications. Ideally one at a time and with testing after each one. If you are also considering moving servers as part of the upgrade, it might be best if you set up DNS aliases for your servers beforehand so the hardware change is transparent to both users and other applications trying to connect.