I use a Garmin Oregon 550T as my bike computer and it works great.. The best choice now is the Garmin Dakota 20 series, a full featured gps but supports garmin bike options like HR and Cadence.. A little smaller form factor over the Oregon series but still much bigger than the 305 or 500 bike garmin series.. aa batteries and long runtime..

https://buy.garmin.com/shop/shop.do?cID=145&pID=30926

The 20 series has a microsd card slot so you can load anytype of maps..

here is a review: http://www.gpsinformation.org/penrod/dakota/dakota.html