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although i'm skeptical of EMA/SMA crossovers following their logic leads me to conclude that depending what time frame you are using the periods for the averages should relate to it
for example, i trade eur/usd on 1m minute charts so i use an EMA of 60 periods because it is an hour
if i was going to use a MA crossover i would, following this reasoning, use a 60 period and a 30 period or perhaps a 60 and 15 (for 1m charts)
for stocks the hour timeframe isn't meaningful so perhaps using the most common periods would lead to the best outcome as a lot of technical analysis is self fulfilling IMO