Perhaps if you searched under "maiai" you would have found
needless to say, 2 cm is not correct maiai
If you can feel their breath, or tell what kind of deodorant they are/are not using... they're too close.
Being "in your face" is an agressive move, now you can't always forsee this, as with anything it comes with training--but you can casually move away as if there wasn't anything more natural.
My sensei always said "circle of death" referring to the point at which the attacker could possible engage us... once that line is crossed, you must have total control of the situation. Remember:
Aikido isn't always physical. It is often mental or verbal.
If someone just "comes up to you" on the street, it is relatively easy to tell if they are going to be 'asking for directions' or if they are going to pose a threat/attack of some kind.
Practice maiai safely in the dojo or with a friend. As always 'perfect practice makes perfect.'
my 2 yen.