The question I have spans the whole trilogy 
What is the size of a unit in Mass Effect? In general, what are the military units in the series?
Does storming the Torfan base with 6 people make sense ?
Unit sizes are never stated in the game or outside sources as far as I'm aware, though it's probably safe to assume that at least for the humans they are organized similarly to modern NATO countries.
The breakdown in ascending order would probably be something like fire team > squad > platoon > company > battalion > regiment > division > corps > army.
Depending on the country or military organization (it varies slightly), a fire team would be 3 or 4 soldiers or Marines, a squad two or three fire teams, a platoon 3 or 4 squads, a company 3 or 4 platoons, a battalion 3 or 4 companies, a regiment 3 or 4 battalions, a division 3 or 4 regiments, a corps 2 to 5 divisions, and 2 to 5 corps to an army. That's a generalization though and sometimes you can have larger units.
All of them can be called units, though typically when someone in the military refers to 'my unit' they mean the company sized unit they are assigned to.
Torfan was a planetary assault so I would think it involved more than just a handful of soldiers. If so I can't imagine that Shepard would be in overall command as he should still be fairly low on the totem pole. More likely he would have been in command of a smaller unit that participated as part of a larger operation involving other units.
Guessing his rank would help guestimate the size of the unit he was in command of. A 2nd Lieutenant for example would typically be in command of a platoon. 1st Lieutenants would typically be the XO for a company and perhaps its acting commander under certain circumstances (like the CO being a casualty). Staff Lieutenant seems to be the Alliance equivalent to an Army or Marine Captain, which would typically be the CO of a company. Lt. Commander seems to be the Alliance equivalent to either an Army or Marine Major or a Lt. Colonel. Majors typically are the XO for a battalion or fill in on certain staff duties for the unit. A Lt. Colonel commands a battalion.
Ruthless Shep was at Torfan six years after joining the military. Depending on how quick (or long) it takes to move up the Alliance chain, he might have been a Staff Lieutenant. My best guess would be that he was a company commander at Torfan. Depending on how the Alliance is organized, a company could be anything from 80 to 250 Marines.