First, in agile teams, there is no assigning of tasks, each member takes the ones they feel like tackling.
Agile in not optimal for maintenance or emergency production issues. These are dealt immediately and only mention in the daily stand up as a blocker stopping you from finish the task you actually intended to complete. They are not even added to the product/sprint backlogs.
As a rule of thumb all user stories added to a sprint should take between half and 2 days to complete. If you have a whole bunch of small bug fixes, group them together so they fit in that time-frame (i.e. all UI bugs in page foo). Otherwise you end up polluting the backlog with hundredths of little things and turning your planning meetings into overwhelming nightmares.
Just think about it: if everybody in the team is working on only one or two tasks every day, the daily stand up will take each member about 30 seconds and the whole meeting less than 5.
