It depends upon the fissionable material, but roughly 11–30lbs. It’s possible to make a backpack weapon of about 50–70 pounds and it’s ‘rumoured’ that the Russians tried to make one or more of them. That would (I am guessing) result in an explosion on the scale of 10–20 Tons of TNT. Which is a lot but it’s not all that impressive. Usually we measure weapons by the kiloton (1000 tons). Furthermore there is a sort of maximum value of practical nuclear weapons, because they get very heavy. So for example, US W-80 cruise missile yields 150kt.
You would need about five of those for Manhattan, and I can’t imagine how many for los angeles (which is a huge territory).