Good point. Most of the folks I know talk about committing 1-2% to any one "idea", taking into account intermarket correlations, which is even more restrictive, e.g. for risk management purposes, going long corn and long wheat would really only be one "idea", not two, as the two are at least somewhat correlated, especially in the case of extreme moves. Similarly, long 30 year and long 10 years would be one "idea", as even though the spread may move around a bit, if there is a huge, ruinous move, they will both be going in the same direction.
Good Trading,
Jessie