There's nothing in the rule book about the base size. The kick starter command squad came with a resin ruined building and the Robot Peanut Studios retail command squad came with a large oval base made by Renedra in the UK, which was to be used for a command diorama with a tent and observation tower. I have both types but have done other command squads using 40 x 45mm Renedra bases (the ones that came with medium field guns and lobototons). I've made some narrower ones (35 x 45mm) so they'll fit in the AQ trenches. I use 4 or 5 figures per small base. The command sets come with 9 figures, 5 of which are infantry at attention.
I've long advocated smaller base sizes for AQ - for the command squad, usually use either a 40x40 mm square base for the 4 figures with a signal tower or a flivver (covered or armored with MG) and three command figures on a 20x20 mm square base with the signal man on top of the flivver (for height). I favor the MG armored flivver for higher order command and having lower level command represented by fighting units (usually some rifle men and a mg). For militia, I've got all those excellent Blue Moon command packs with a pistol officer and two different rifle armed sergeants. Add in a LMG (from the MG pack with 8 LMG and 8 MG) and you've got company & platoon level figures. The 5 standing at attention figures are usually combined to form a full squad at attention awaiting deployment or in a trench line to the rear of the battle, usually in from of HFG gun emplacements as they've usually not engaged right away. They also do well in ruined building before battle. As you take casualties, I swap out the attention figures with 'fighting' ones. All my two cents worth - remember that I usually put squads on 40x20 & 20x20 mm bases to make up 100 mm x 20 mm so they both fit better in most terrain and give a more flexible approach to game use. Just my 2 cents worth
Like Morgan I've used most of the 'attention' figures from the HQ sets to form an infantry platoon that's 'in reserve' near the Field Commander. I used 3 of them on a large oval base HQ diorama but never more than 1 on any other command bases.