Net BIOS

I don't understand the relation between title and question, but here goes.
If you implement an AD tree, you first look at your organization. Most companies choose to make Organizational Units (OU) in AD according to departemental lines. Those OU can be set up granular within a single domain, or in several subdomains.
I assume that those workgroups you mention are operational units within the domain. This way you can assign administrative rights on a workgrouplevel, while you still have central control.
The size of the workgroups has no impact on the performance of AD. However the reliability of the wan connections is far more important. It is advisable to place a domain controller behind each wanlink, with the master domain controller on a central location. This is more or less like the old PDC/BDC model.