It's better to start with figures that are relatively high, and which can cope with a long run of bad luck (say a month, rather than just a day or 2). If you start with too small a bankroll, you can just bust out.
Then again, if you're perfectly able and willing to redeposit, then you can start with a smaller amount, because it's not the total limit of your available funds.
One other thing to bear in mind, though, is that most of the poker sites
deposit bonuses are a % of the amount you deposit, so it may be worth looking at how much you need to deposit to get the maximum return (and additionally how much you need to play in what period of time to clear the bonus, to make sure you don't have a bonus that expires before you earn it)