Straingages have alignment marks on them to centre them precisely at a point on a test part, but they respond to strain over the area covered be the sensitive grid, due to which the reading from the straingage will be an average of all the strain values under the whole grid.
If the test part has a constant strain field, ( as in the case of a bar of uniform cross section loaded in tension or compression), or a linearly changing strain field (as in the case of a cantilever beam in bending), the the average of all strain values under the grid of a straingage will be the same as the strain at the centre of the grid,
and the size of the straingage used is not important.

When there is a strain gradient or a non-linear strain field on the test part, the peak strain at the point of maximum stress is of interest. In order to reduce the averaging effect of the strain gage, it is important to use a gage with a small grid covering a small area, and centre it on the point of interest, to get an output as close to the peak strain as possible.