Ground is the same as 0V- technically speaking there isnt a battery negative, just a battery 0V, unless of course youve got 2 batteries wired up to give a +ve, -ve, and 0V, but thats another story...

The R values are fairly arbitary- but those are fairly standard for voltage driven devices- they are pull down resistors, which stop the ouput device floating, which may turn the MOSFET on, even when you think its off, which obviously isnt particularly desirable. These are reletively high R values, but thats of no consequence in a voltage driven device, in fact its more desirable. Its normally just experience through playing and tinkering and designing such circuits- no real calculations as such.