First off: you can start tiny with a bankroll size of about 10 BTC, where you allow a max win of .25% of the bankroll: 0.025BTC.
If you want to compete with the larger guys, and have a max win of around 10BTC (or maybe ever larger), and assuming you don't want to risk too much of your bankroll, you will need at least a 4000 BTC bankroll.
Also, don't forget that the amount is probably even higher, because there are all kind of expences that you have to pay.
This feature to set a max winning percentage of the total bankroll is very important to prevent losses you can't afford to pay people who are gambling on your site, principally at the beginning, when the bankroll is usually lower.
Try to have biggest bankroll as possible to attract gamblers that like to bet high and consequently spend more money on your site, these guys give some nice profit for the house when playing. The little fishes play low and don't generate much profit. I agree that 0.25% of total bankroll maximum bet is adviced.
Anyway, to have a big bankroll isn't a guarantee to have success. There are another skills very necessary like marketing and management.