Join Date: Aug 2009
Location: Portland, Oregon
From what I've been able to gather, there's usually 10 tanks to a company. That probably means 60 men to a company. Say, about 15 companies makes up a regiment (at least in Infantry regiments). That's a base of 900 men.
Then you need on-hand replacement crews, mechanics, priests (bot of the spiritual and Mechanicus side), support crew manning the atlases, and the like and you could probably end up around 1500-1800 men in the least.
The Imperial Guard Codex mentions an armored regiment toward its start that says it only had 1500 men. In the context of the Codex, however, this was pitched as a small regiment. Then again, it may just be low when compared to standard infantry regiments. It goes to say Super-Heavy regiments number only a dozen machines and those are probably the smallest regiments in terms of head counts. So I don't think that this 1500 number is the smallest possible.
Given that this unit you're making is at full strength, I'd probably pitch somewhere around 900-2000 men. This depends on how many companies of the regiment are manned by 6 manned tanks, 10 company tanks, versus less men-intensive companies of artillery, sentinels, ect.