I have a model 1915 kurassier helmet lacking the spike. My question is should I be looking for a special steel spike (from some of the photos I have seen on line, they seem longer) or would a standard spike be correct.
I think that you would be safe with either. Some manufacturers (Lachmann for example) made M15 removable spikes that looked like the M1894 Kürassier spikes. However, some, Juncker for example) had spikes that appear to be the standard M15 for all branches. As far as height, you will find height differences even on infantry examples; some are very tall. So this also is most likely a manufacturer variation.