You can
- get the screen sizes for the devices you plan to support,
- Create say, 2-3 versions of your site and detect useragent string and direct user to one of these sites.
With android, iphone and blackerry devices there is not large difference in a screen sizes. With a litle foresight and design you may be able to have all these handset work on one mobile website (for example it would be fair to say that the width would be 240 pixels or greater and design accordingly)
However if you plan to go further than there these platforms there is a world of hurt awaiting you, (J2ME, BREW, Symbian, WinMo and 120x160, 176x220 screen sizes etc)