max # of characters for links to work in emails?

hi, i'm working on an eCard project and somehow need to find out what the reasonable limit is on characters for urls in email bodies (normal text, not mime). i know that some mail clients will cut off urls that are too long or else throw a line break in the middle of them--thus rendering them 'unclickable'. and this is exactly what i wanna avoid. anyone have any advice to offer in this area? muchos gracias, tom