Consideration of the smallest division of the measuring device leads to a best estimate of the uncertainty. However, in this situation the uncertainty may be larger (due to the difficulty in determining the endpoints of the length) in which case you should use your judgement to estimate a reasonable value.

You must use 1 mm.

You must use 0.5 mm.

As you have only taken a single measurement, it is not possible to determine the uncertainty in this situation and so you should not state one.

You must use 10% of the measured value.