Diodes in series, 3kV
I think the idea is that the voltage divider formed by the parallel resistors dominates the 'divider' formed by the reverse biased diodes. So in other words, you want considerably more current through the resistors than the diode's typical reverse bias current. Put another way, smaller resistors will do a better job of balancing the reverse biased diode drops. Of course you don't want too much leakage current because this will bring down the average voltage you can attain, so as usual there's a trade off to be made. 

Also, you probably already know this, but don't try to get 1kV of drop across a single 1/8 watt sized resistor - it's better to series up a few smaller ohm values to get the resistance you want, so as to physically spread the drop out and reduce the E field. Might not matter too much until you get into 10's of kV but steep E fields cause corona.