Because it's the desert, base size isn't an issue in most circumstances (no hedged roads, etc.). Consequently, I decided base sizes in the following, completely arbitrary, manner.
I lined tanks up as close as possible without making them look like a 'wall', 'walls' of tanks being something I wanted to avoid. It turned out to be about 60mm a vehicle - which, base to base, gives a gap of about 35mm between vehicles. It was adopted as the standard base frontage for everything. Base depth varies - 75mm, 90mm, 60mm and 40mm depending on what it is.
Depth was decided by the longest tank, which in this case was a Crusader 1, so I adopted a general vehicle base depth of 75mm. Only the Sdkfz 9 needs a 90mm depth base, even motocycle combinations are on 75mm (2 to 3 models a base).
Guns are on 75mm, 90mm or 60mm depth depending on the model. (BTW, the 6mm red guns are off table battery markers. They sit in a little wooden box at the table edge as aide de memoire - it's quicker than writing a roster and more easy to mark as fired, etc.)
I did the infantry on 40mm depth because it looks about right. Also pictured a Btn HQ (75mm diameter) and a 2pdr on a 60 x 60mm base.
So that HQs stand out I did them on round bases. Battalion (Brit Regt) usually one vehicle are 75mm diameter; Brigade (German Regt) usually two vehicles are on a 90mm diameter, and each side has a Divisional / Group HQ on a 115mm diameter. FAO are on 60mm.
A German Panzer Regiment showing 2 Btn HQs and a 'Brigade' HQ:
A British Divisional HQ
FAO on a 60mm
So, no logic really. I just didn't want to ever see tanks lined up track to track, and a wide base stops it happening.