My front yard is 30' x 50' (1500 sqare ft), and I'm wondering what size K-Rain rotors should I use (K1's or K2's)?; How many do I need?; and Where should I place them (in a Cross or in the corners of the lawn?
Since your profile states you are a homeowner, I would suggest 2 things first and easiest call a contractor in your area and get a quote. Second if you are set on doing it yourself read all the books you can on the subject.
I strongly suggest that you hire a professional , and go with the best , not the cheapest. It will be less expensive in the long run.
There are a few factors which should be considered. The radius of the heads will be dependent upon what your water pressure is, and the amount of water you have available would determine if you need more than one zone. For example, if you have six heads each with 3.0 gallon per minute nozzles on them, that equals a total of 18 gallons per minute. If you only have 15 gallons per minute available then something has to give, one way or another. Do you know what size and type of pipe to use? Does the wind blow often and are there trees in the yard?
There's more than just picking out sprinkler heads, and to do it correctly you need to figure everything into the equation... or like Mdirrigation said, get a professional to do it.
Again, I wouldn't assume anything. The configuration may be correct but make sure you have enough water when you add up all the gpm of all of these nozzles. My own home had to be designed at 10 gpm. That gave me a little room for flushing toilets or taking showers while the irrigation is running. Four 3.0 nozzles in the corner plus two 4.0 nozzles along the 50' side requires 20 gpm. Thats quite a bit of water, and you better add a bit more for toilets, etc. Also, 20 gpm through a 1" main line is even a bit much for that size of pipe, and you would have almost 5 psi pressure loss. Go with two zones, same configuration, that is if you have enough pressure to handle the 30' throws.
I'm not familuiar with K-Rain rotors. But, assuming using Rainbird 5000 rotors at 25 psi. 3 gpm nozzles put out 2.58gpm and have a precip rate of .41 in/h. To get 4.2 gpm you would need a 6.0 gpm nozzle and get .7 in/h. 1/2 the size of the corner nozzle.