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What is a large contract size in YM or ZB

Ok,no funny business here guys,i'm not talking downstairs.
What kind of size is worth taking note of in the YM and ZB?
In other words if i was watching the tape in either of these markets,what would make me go..."Wow,that's BIG!!".
I realise that in certain market conditions there may be a considerable spike to the average,but in terms of 'normal' market conditions what should i take note of.
I appreciate any comments anyone can offer.