If you're going to race events covered by BMXA then section 18 of the UCI and ARA Rule Book 2008 states:
J) Handlebars must not exceed 735mm (29 inches) in width for both 508mm (20 inch) bicycle and Cruisers, and must have both ends plugged. Measurement includes handle grips.
Of course if you're not racing with them then as wide as you feel comfortable with I guess.