Measuring the lengths of curved spores sometimes makes confusions. Here in the attached image I have marked two paths (i, ii) the length can be measured. Hope an expert can suggest the correct way.
Method ii is the most representative and applicable method, as method i is very subjective and will vary depending on where you draw the line. The key point here is to measure many spores and get a representative average with some measure of the variation (standard error or standard deviation). As long as you disclose your chosen method there is no issue in using either, but method ii is preferred, as it is easier to replicate.
I think method ii is more reliable. Curvature will differ among different species. So, if you are dealing with many samples, it is important to ensure uniformity. In such case, you may go with method ii straight line measurement.