<<First, the actual existence of "suitcase" nukes is disputed. While it may be a good exercise to entertain their existence, it is not a foregone conclusion that they do in fact exist.>>
The weight of such a device would be immense. A nuke needs a minimum of 67kg (147lbs) of Plutonium for critical mass, this can be reducded a bit with a very high explosive implosion before chain-reaction occurs. If you don't want someone to die after being within 20' of the thing for 5seconds you would need some solid lead sheilding. Depending on if you wanted to kill the guy delivering it, you would need probably between 4-12inches of lead around the explosive core. Add in the weight of timing devices, explosives to start the reaction and other misc items and you are probably looking at around 500lbs minimum. If you see anyone dragging a suitcase around that weighs 500lbs, get worried, otherwise, don't.