Strain gauges measure the average strain over the area covered by the sensitive grid lines, and not the strain at a single point at the centre of the grid. The average value of strain produced by the strain gauge will depend on the nature of the strain field under it. If the strain field is constant or varies linearly, the strain value will be equal to the strain on the part at the centre of the strain gage, regardless of the size of the gage. However if the strain field is non-linear or has a steep gradient, then a small strain gauge should be bonded at the location of maximum strain, to localize the averaging effect as much as possible and to get a strain value close to the peak strain.